In this study, a near-isogenic line (BC4F10) CR572 developed by introgressing a chromosomal segment from Oryza rufipogon (accession no. 105491) into the Oryza sativa subsp. japonica cv. Hwaseong was found to exhibit a significant increase in the number of spikelets per panicle (SPP) and grain weight compared with the recurrent parent Hwaseong. Quantitative trait locus (QTL) analysis in F2 generation derived from the cross between CR572 and Hwaseong revealed that two QTLs, qSPP1 and qTGW1, were linked to a simple sequence repeat marker, RM283, on chromosome 1. The additive effect of the O. rufipogon allele at qSPP1 was 13 SPP, and 21.6% of the phenotypic variance was explained by the segregation of RM283. The qTGW1 QTL explained 19.1% of the phenotypic variance for grain weight. Substitution mapping was carried out with five F3 lines derived from F2 plants having informative recombination breakpoints within the target region. Substitution mapping indicated the linkage of qSPP1 and qTGW1. The grain yield of CR572 was 18.2 and 15.8% higher than that of Hwaseong at two locations, respectively, mainly due to the increase in 1000-grain weight and SPP. These results are very useful for QTL cluster transfer by molecular marker-assisted selection in rice breeding programmes and for QTL gene cloning by map-based cloning.