Abstract: The yield of unstable chromosome aberration induced in human lymphocytes by β particles from low-doses HTO has been measured. HTO was mixed with heparinised blood in various amounts so that dose of 6.24×10^-4 Gy to 1.23 Gy were delivered in 24 h and 48 h. After culturing for 72 h, the dicentric yield was measured as a function of dose to the blood and compared with data from ⁶⁰Co γ radiation. Using a linear-quadratic dose-effect relation to fit the experimental data, a significant linear contribution Y=(0.001±0.004)+(0.062±0.018)D+(0.053±0.010)D²（n=3，r²=0.995, P<0.01） was found. The main difference between the coefficients for β and γ radiation is in the b values, indicating that HTO β rays are more efficient, particularly at low doses. In accord with the theory of dual radiation, the RBE of HTO β particles relative to γ rays is 2.17 at 0.06 Gy and decreases with increasing dose.