Abstract: We investigated the cytological characteristics and hormone responses of semi-dwarf rice mutant B1-24 by comparing to the wild type Huiyangzhenzhuzao (Hyz). Compared with Hyz, the height of B1-24 seedlings grown in light was 14.62% shorter and the length of the first leaves was 11.18% shorter; while the mesocotyl length of B1-24 was 41.89% longer but, the coleoptile of Hyz was 28.71% longer when cultured in darkness. Further observation showed that there were no significant difference between the cell lengths or width of first leaves from B1-24 and Hyz seedlings. In addition, the lengths of coleoptile and mesocotyl cells of etiolated B1-24 seedlings were 45.95% and 118.28% of that of Hyz, respectively. Treated with 600 μg/ml GA3 or 7 μg/ml IAA, the cell length of B1-24 was significantly increased to the same level of Hyz. While 50 μg/ml PAC could severely shorten all of them. Our results suggested that the semi-dwarf phenotypes of B1-24 may be related to the synthesis of, or response to phytohormone(s).