Abstract: A thermo-sensitive leaf-color mutant, mr21, was discovered in progeny of a rice japonica variety, Jiahua No.1, treated by ⁶⁰Co γ-radiation. Under low temperatures (<25.0°C), the mutant’s seedling leaf color was yellow and turned green with an increase in temperature; the critical temperature for leaf-color alteration was about 27.5°C. As well, the contents of total chlorophyll and chlorophyll a and b in mr21 mutant were decreased compared with the wild-type at low temperatures, which showed that the character of leaf color was sensitive to temperature. Genetic analysis revealed that leaf color was controlled by a single recessive nuclear gene, tentatively designated thermo-sensitive leaf-color 1 (tsl-1). Using the F₂ population derived from the cross 9311×mr21 and simple sequence repeat markers, we preliminarily mapped the tsl-1 gene between MM1799 and RM8132 on the short arm of chromosome 1, with a genetic distance of 2.4 and 3.0 cM, respectively. With further development of new molecule markers and a larger F₂ mapping population, the tsl-1 gene was located between InDel 2 and InDel 4 molecular markers with the physical distance of 198 kb. The results will be useful for cloning and functional analysis of the tsl-1 gene.