Grain shape (grain length, width, and thickness) determines crop yield and grain quality. However, the trait is regulated by numerous naturally occurring quantitative trait loci (QTLs) and the underlying mechanism remains largely unknown. Here, we report the genetic mapping of three new QTLs, qLG2, qWG2, and qLG8 that each exerts a semi-dominant effect on grain shape in cultivated rice. These QTLs were validated using populations derived from the corresponding chromosome segment substitution lines (CSSLs), and were further delimited to small genomic intervals in progeny testing experiments. Especially, qLG2/qWG2 was placed into an about 250-kb genomic candidate region, and 14 predicted ORFs localized within the interval. We also evaluated the individual and pyramiding genetic effect(s) of these QTL(s) using the corresponding nearly isogenic lines, and found that they have additive effects on the traits. Collectively, these findings provided useful information as a tool to improve grain shape in crop breeding programs and established foundations for future QTL cloning.