Leaf color mutants are often accompanied by changes in chlorophyll content and abnormal chloroplast structure, and serve as essential materials for studying the functions of chloroplast development and photosynthesis-related genes. In this study, we obtained a yellow-green leaf mutant named yellow-green leaf 18 (ygl18) from Oryza sativa subsp. indica cv. ‘HZ’ with ethyl methanesulfonate (EMS). Compared with the wild type, the leaves of ygl18 turned yellow at three-leaf stage and the degree of yellowing increased as it grew, accompanied by decreasing photosynthetic rate and chlorophyll content. The seed-setting rate, 1 000-grain weight, and effective panicle number were significantly lower than those of the wild type. We observed disordered chloroplast structure, loose stromal lamellas, and stalled development in the mutant using transmission electron microscopy. Genetic analysis indicated that the mutant feature (or phenotype) of ygl18 is controlled by a pair of recessive nuclear alleles, which were located in a 115.2 kb region between markers InDel2 and InDel3 on the long arm of chromosome 3. We found mutations in the 5′UTR of LOC_Os03g48040 encoding FdC2 (ferredoxin C2). The gene’s function on controlling the mutant phenotype was verified using CRISPR transgenic experiments. Our results revealed a genetic basis for leaf color regulatory network and provide new clues for breeding photosynthetically efficient rice varieties in the future.