To clarify the most appropriate sample size for obtaining phenotypic data for a single line,we investigated the main-effect QTL(M-QTL) of a quantitative trait plant height(ph) in a recombinant inbred line(RIL) population of rice(derived from the cross between Xieqingzao B and Zhonghui 9308) using five individual plants in 2006 and 2009.Twenty-six ph phenotypic datasets from the completely random combinations of 2,3,4,and 5 plants in a single line,and five ph phenotypic datasets from five individual plants were used to detect the QTLs.Fifteen M-QTLs were detected by 1 to 31 datasets.Of these,qph7a was detected repeatedly by all the 31 ph datasets in 2006 and explained 11.67% to 23.93% of phenotypic variation;qph3 was detected repeatedly by all the 31 datasets and explained 5.21% to 7.93% and 11.51% to 24.46% of phenotypic variance in 2006 and 2009,respectively.The results indicate that the M-QTL for a quantitative trait could be detected repeatedly by the phenotypic values from 5 individual plants and 26 sets of completely random combinations of phenotypic data within a single line in an RIL population under different environments.The sample size for a single line of the RIL population did not affect the efficiency for identification of stably expressed M-QTLs. To clarify the most appropriate sample size for obtaining phenotypic data for a single line, we investigated the main-effect QTL (M-QTL) of a quantitative trait plant height (ph) in a recombinant in line line (RIL) population of rice from the cross between Xieqingzao B and Zhonghui 9308) using five individual plants in 2006 and 2009. twenty-six ph phenotypic datasets from the completely random combinations of 2,3,4, and 5 plants in a single line, and five ph phenotypic datasets from five individual plants were used to detect the QTLs.Fifteen M-QTLs were detected by 1 to 31 datasets. Of these, qph7a was detected repeatedly by all the 31 ph datasets in 2006 and explained 11.67% to 23.93% of phenotypic variations; qph3 was detected repeatedly by all the 31 datasets and explained 5.21% to 7.93% and 11.51% to 24.46% of phenotypic variance in 2006 and 2009, respectively. The results indicate that the M-QTL for a quantitative trait could be detected repeatedly by the phenotypic values ​​from 5 indivi dual plants and 26 sets of completely random combinations of phenotypic data within a single line in an RIL population under different environments. sample size for a single line of the RIL population did not affect the efficiency for identification of stably expressed M-QTLs.