Background. Grain weight is a grain yield component, which is an integrated index ofgrain length, width and thickness. They are controlled by a large number of quantitativetrait loci (QTLs). Besides major QTLs, minor QTLs play an essential role. In ourprevious studies, QTL analysis for grain length and width was performed using arecombinant inbred line population derived from rice cross TQ/IRBB lines. Two majorQTLs were detected, which were located in proximity to GS3 and GW5 that have been cloned. In the present study, QTLs for grain weight and shape were identified using rice populations that were homozygous at GS3 and GW5.

Method. Nine populations derived from the indica rice cross TQ/IRBB52 were used.An F_10:11 population named W1, consisting of 250 families and covering 16 segregating regions, was developed from one residual heterozygote (RH) in the F₇ generation of Teqing/IRBB52. Three near isogenic line (NIL)-F₂ populations, ZH1, ZH2 and ZH3 that comprised 205, 239 and 234 plants, respectively, were derived from three RHs in F10:11. They segregated the target QTL region in an isogenic background. Two NILpopulations, HY2 and HY3, were respectively produced from homozygous progeny ofthe ZH2 and ZH3 populations. Three other NIL-F₂ populations, Z1, Z2 and Z3, wereestablished using three RHs having smaller heterozygous segments. QTL analysis for1000-grain weight (TGW), grain length (GL), grain width (GW), and length/width ratio(LWR) was conducted using QTL IciMapping and SAS procedure with GLM model.Result. A total of 27 QTLs distributed on 12 chromosomes were identified. OneQTL cluster, qTGW2/qGL2/qGW2 located in the terminal region of chromosome2, were selected for further analysis. Two linked QTLs were separated in regionTw31911−RM266. qGL2 was located in Tw31911−Tw32437 and mainly controlledGL and GW. The effects were larger on GL than on GW and the allelic directions wereopposite. qTGW2 was located in Tw35293−RM266 and affected TGW, GL and GWwith the same allelic direction. Finally, qTGW2 was delimited within a 103-kb regionflanked by Tw35293 and Tw35395.Conclusion. qTGW2 with significant effects on TGW, GL and GW was validated andfine-mapped using NIL and NIL-F2 populations. These results provide a basis for mapbased cloning of qTGW2 and utilization of qTGW2 in the breeding of high-yielding ricevarieties.