Abstract: In order to facilitate intravenous injection of mice, directive breeding on short tail and clear tail vein of KM mice was done for 4 generations by using rotational mating system, and their tail length, growth and reproductive performance were measured and observed. The results showed that the average tail length of the short tail KM mice after directive breeding was 67.5±6.0mm at 4 weeks old，shorter than that(80.3±12.6mm，P＜0.01) of the control ones (KM mice) , while no significant differences were found in body weights, litter size and weanling number. The rate of the mice with clear tail vein in short tail KM mice(n= 200) was 95% while in control ones(n=160) 20%.