In a previous study, we reported the grain weight QTL, tgw2 in the 150 $F_{2:3}$ lines derived from a cross between Oryza sativa subssp. Japonica cv. Hwaseongbyeo and HG101. This QTL was confirmed in $F_4$ lines (CR1242) segregating for the target region. For fine mapping of tgw2, one $F_5$ plant homozygous for the O. grandiglumis DNA in the target region was selected from CR1242 and crossed with Hwaseongbyeo to produce the $F_2$ and $F_3$ populations. QTL analysis using 490 $F_2$ plants confirmed the existence of tgw2 with an $R^2$ value of 28.0%. This QTL explained 61.3% of the phenotypic variance for 1,000-grain weight in 64 $F_3$ lines. Substitution mapping with 47 $F_3$ lines and 74 $F_4$ plants with informative recombination breakpoints in the target region was carried out to narrow down the position of the tgw2. The result indicated that tgw2 was located in the 384-kb interval between two SSR markers, RM12813 and RM12836. Annotation data of BACs in this 384-kb region revealed that forty-five putative genes exist in this interval including the GW2 gene responsible for grain weight and width. Considering the position of the QTL tgw2, it appears that tgw2 is functionally related to the gene GW2. However, the possibility that another unknown mechanism might be responsible for regulation of grain weight at tgw2 cannot be ruled out. Four QTLs for grain length, grain width, and grain thickness were also located in the same interval suggesting that a single gene is involved in controlling these four traits. Substitution mapping also indicated that two QTLs for grain weight and culm length, tgw2 and cl2, were tightly linked.