A set of chromosome segment substitution lines (CSSLs), derived from Asominori/IR24 with Asominori as the recurrent parent,was planted and phenotyped for grain length (GL), grain width (GW) and length-width ratio (LWR) of milled rice in two successive years and four sites. QTL for GL,GW and LWR were characterized and stability of their expression was investigated. The phenotypic values for each trait showed a continuous distribution and some transgressive lines were also observed in the CSSLs population. Additionally, a total of 13 QTL for GL, GW and LWR were identified,and six of them were consistently detected in the eight different environments. Phenotypic values were different significantly (P < 0. 001) between the CSSLs harboring any of the six QTL alleles and the genetic background parent, Asominori. Significant phenotypic correlations (r > or = 0.75, r0.05 = 0.67) were detected among different environments for these CSSLs carrying the same target QTL. Also, the results indicated that the six QTL, namely, qGL-3 for GL, qGW-5a and qGW-5b for GW, qLWR-3, qLWR-5a and qLWR-5b for LWR, were stably expressed in different environments. Since QTL qGL-3 and qLWR-3 were mapped in the R19-C1677 interval, qGW-5a and qLWR-5a in the vicinity of RFLP marker C263, qGW-5b and qLWR-5b near R569,the four RFLP markers, R19, C1677, C263 and R569,would be useful for further marker-assisted selection (MAS) in rice quality improvement.