This study examines the relationship among cooling rate,microstructure and mechanical properties of a sand-casted A1-5.0Mg--0.6Mn-0.25Ce (wt％) alloy subjected to T4 heat treatment (430 ℃ × 12 h + natural aging for 5 days),and the tested alloys with wall thickness varying from 5 to 50 mm were prepared.The results show that as the cooling rate increases from 0.22 to 7.65 K/s,the average secondary dendritic arm spacing (SDAS,λ2) decreases from 94.8 to 27.3 μm.The relation between SDAS and cooling rate can be expressed by an equation:λ2 =53.0R-0.345c.Additionally,an increase in cooling rate was shown not only to reduce the amount of the secondary phases,but also to promote the transition from Al10Mn2Ce to α-A124(Mn,Fe)6Si2 phase.Tensile tests show that as the cooling rate increases from 0.22 to 7.65 K/s,the ultimate tensile strength (UTS) increases from 146.3 to 241.0 MPa and the elongation (EL) increases sharply from 4.4 to 12.2％ for the ascast alloys.Relations of UTS and EL with SDAS were determined,and both the UTS and EL increase linearly with (1/λ2)0.5 and that these changes can be explained by strengthening mechanisms.Most eutectic Al3Mg2 phases were dissolved during T4 treatment,which in turn further improve the YS,UTS and EL.However,the increment percent of YS,UTS and EL is affected by the cooling rate.