An abnormal organs mutant big size organ1 (bso1), was isolated by enthylmethane sulfonate (EMS) mutagenesis strategy. Genetic analysis indicated that the mutation phenotype was controlled by one recessive nuclear gene. Compared to the wildtype, bso1 showed significant increase in seedling, flowers, pods and seeds. The detection in paraffin sections of seeds displayed that the increase of the bso1 seeds primarily due to the increase of germ cell, so the seeds of bso1 is heavier than that of wild type. The mutation loci was mapped to the region of 58kb between SSLP marker T5L19 and F28M11 on Chr.4. According to the result of bioinformatics analysis, there are no previous genes about organ development in the region. The results provide a basis for further cloning and functional analysis of bso1 in organ development, especially in seed development.