Objective To clarify the G banding pattern and spontaneous aberration frequencies of a new inbred line mouse IRM 2. Methods The bone marrow chromosome preparation and G banding pattern for mouse were applied.Results Of the 50 G banding cells from 20 mice,10 cells were selected for banding analysis. The chromosomes were distinguished by characteristic banding patterns. The relative length and standard deviation were depicted. Some discerned points between similar banding of chromosomes were presented, such as 1, 6 and X, 4 and 5, 9, 13 and 14. In addition, 1200 metaphase cells were analysed by conventional Giemas stain. The results showed that break was 0.33%, acentric aberration and translocation were 0.08%. The spon taneous aberration frequencies were very low. Conclusion The discernment of G banding pattern of a new inbred line mouse IRM 2 is for providing scientific basis to study on abnormal structure, radiation effect, cancer research and gene map.