Extreme high temperatures detrimental to maize production are projected to occur more frequently with future climate change.Phenology and yield-related traits were investigated under several levels of elevated temperature in two early-maturing hybrid cultivars:Junda 6(grown in northeastern China)and Chalok 1(grown in South Korea).They were cultivated in plastic houses in Suwon,Korea(37.27°N,126.99°E)held at target temperatures of ambient(AT),AT+1.5°C,AT+3°C,and AT+5°C at one sowing date in 2013 and three different sowing dates in 2014.Vegetative and reproductive growth durations showed variation depending on sowing date,experimental year,and cultivar.Growth duration tended to decrease,but not necessarily,with temperature elevation,but somewhat increased again above a certain temperature.High temperature-dependent variation was greater during grain filling than in the vegetative period before anthesis.Elevated temperature showed no significant effects on duration or peak dates of silking and anthesis,and thus on anthesis–silking interval.Grain yield tended to decrease with temperature elevation above ambient,showing a sharper linear decrease with mean growing season temperature increase in Junda 6 than in Chalok 1.The decrease in kernel number accounted for a much greater contribution to the yield reductions due to temperature elevation than did the decrease in individual kernel weight in both cultivars.Individual harvestable kernel weight was not significantly affected by temperature elevation treatments.Kernel number showed a linear decrease with mean growth temperature from early ear formation to early grain-filling stage,with Junda 6 showing a much severer decrease than Chalok 1.Kernel number reduction due to temperature elevation was attributable more to the decrease in differentiated ovule number than to the decrease in kernel set in Chalok 1,but largely to the decrease of kernel set in Junda 6. Extreme high temperatures detrimental to maize production are projected to occur more frequently with future climate change. Phenology and yield-related traits were investigated under several levels of elevated temperature in two early-maturing hybrid cultivars: Junda 6 (grown in northeastern China) and Chalok 1 grown in South Korea. They were cultivated in plastic houses in Suwon, Korea (37.27 ° N, 126.99 ° E) held at target temperatures of ambient (AT), AT + 1.5 ° C, AT + 3 ° C, and AT + 5 ° C at one sowing date in 2013 and three different sowing dates in 2014. Vengetive and reproductive growth durations showed change variation on sowing date, experimental year, and cultivar. Growth duration tended to decrease, but not necessarily, with temperature elevation , but somewhat increased again above a certain temperature. High temperature-dependent variation was greater during grain filling than in the vegetative period before anthesis. Elevated temperature shows no significant effects on duration or peak dates of s ilking and anthesis, and thus on anthesis-silking interval. Grain yield tended to decrease with temperature elevation above ambient, showing a sharper linear decrease with mean growing season temperature increase in Junda 6 than in Chalok 1. The decrease in kernel number above for ambient much greater contribution to the yield reductions due to temperature elevation than than the decrease in individual kernel weight in both cultivars. Individual harvestable kernel weight was not significantly affected by temperature elevation treatments. Kernel number showed a linear decrease to early grain-filling stage, with Junda 6 showing a much severer decrease than Chalok 1. Kernel number reduction due to temperature elevation was attributable to the decrease in differentiated ovule number than to the decrease in kernel set in Chalok 1, but largely to the decrease of kernel set in Junda 6.