Grain weight is the most important target not only as a major component of grain yield, but also of the cooking qualities in rice breeding program. In a previous study, a high-resolution physical map targeting a cluster of yield-related QTLs for grain weight, spikelets per panicle has been constructed using series of BC3F4 nearly isogenic lines (NILs) derived from a cross between the Korean japonica cultivar Hwaseong and O. rufipogon. The QTLs including grain weight trait have been mapped in a 25.5kb region containing three genes. Based on GenBank database, these genes include male sterility 5 (OsMs5, LOC_Os09g36740), similar ascorbate peroxidase (OsApx, LOC_Os09g36750) and glutelin family protein (OsGlu, LOC_Os09g36760). Their endogenous expression patterns were analyzed in various rice tissues (2-week seedling, flag leaf, root and panicle) from the parental lines, Hwaseong and NIL-gw9. Semi-quantitative RT-PCR and qRT-PCR were performed using gene specific primer sets. The cDNAs of the similar OsApx gene of Hwaweong and NIL-gw9 were cloned. Over-expression and RNAi knock-down transgenic plants using three genes are under construction for the functional characterization of the genes. The results will be discussed.