Leaf size is closely related to photosynthesis and greatly affects rice productivity. Therefore, the search for quantitative trait Locus (QTL) that regulates leaf size is important for improving productivity through rice breeding. The F₉ recombinant inbred line (RIL) population from the cross between ‘Unbong 40’ (large leaves) and ‘Odae’ (ordinary size leaves) was grown in a test field, and the size traits of the flag leaves and 2nd leaves, along with stem and panicle length, were measured. Through QTL analysis of these traits, we detected 20 QTLs on chromosomes 1, 3, 6, 9, 11, and 12. In particular, the flag leaf length QTL qFLL3, the flag leaf area QTL qFLA3, the 2nd leaf length QTL q2LL3-2, and the 2nd leaf area QTL q2LA3 were clustered in the 149.0-161.6 cM region of chromosome 3, indicating that one QTL gene in this region may have pleiotropic effects that regulate the flag leaf length, flag leaf area, 2nd leaf length, and 2nd leaf area. In addition, the flag leaf width QTL qFLW9 and 2nd leaf width QTL q2LW9 were clustered in the 2.1-22.86 cM region on chromosome 9, which suggests that one QTL gene in this region may have pleiotropic effects that regulate both the flag leaf width and 2nd leaf width. These results serve as a valuable reference for breeding programs aiming to increase photosynthesis and yield by identifying QTLs for leaf size traits in Korean japonica rice.