A spontaneous mutant showing yellow-green leaf throughout the growth period, designated as ygl11(t), was screened and identified from rice cultivar Nanjing 41. This mutant was grown for several generations and showed stable mutation phenotype. The ygl11(t) mutant has reduced chlorophyll accumulation and abnormal chloroplast development. The average contents of total chlorophyll, chlorophyll a, chlorophyll b, and carotene in the ygl11(t) mutant were 45.7% to 74.7%, 55.2% to 87.5%, 12.5% to 25.3%, and 62.3% to 97.0% of these in wild type, respectively, at the stages of seedling, tillering and heading. The leaf net photosynthetic rate of the ygl11(t) was significantly higher at maximum tillering stage and slightly lower at 10 days after flowering than that of wild type. Ultrastructure analysis revealed that the grana lamellae of chloroplast thylakoid were less in number and abnormally developed in the ygl11(t) mutant. Genetic analysis suggested that the phenotype of ygl11(t) was controlled by a recessive nuclear gene. YGL11(t)was prelimarily mapped on the long arm of rice chromosome 10, and was finally narrowed to a 58.1 kb region containing six open reading frames (ORFs). All the six ORFs were amplified and sequenced. A 2-bp deletion was found in the 9th exon of the OsCAO1gene, encoding a chlorophyll a oxygenase 1, which results in a premature stop codon and protein truncation in the ygl11(t) mutant. These results suggest that the OsCAO1 gene is the candidate of YGL11(t).