The mouse melanoma cells (B16), human cervical squamous carcinoma cells (HeLa), Chinese hamster pulmonary cells V79, and human hepatoma cells (SMMC-7721) were collected for studing. The cells of 5 x 10(5)/ml were seeded in 35 mm diameter petri dish and allowed to grow one day, and then the medium in petri dishes was removed away, the cells were washed once with phosphate-bufferd saline (PBS), petri dishes was covered with 4mum thickness Mylar film. The cells were irradiated by C-12 ion beam with LETs of 125.5, 200, 700 keV/mum in water generated from HIRFL ( Heavy Ion Research Facility in Lanzhou). For Co-60 gamma-ray experiment, the cells of 5 x 10(4)/ml were grown in 20ml culture flasks including 1.5ml cell suspension and directly used for irradiation. Following irradiation, the cells were trypsinized, counted, plated at appropriate densities in growth medium and then seeded in 60mm diameter culture dishes. Each dish was filled 4ml standard medium, and incubated for 8-12 days at 37degreesC incubator containing 5% CO2. The cultures were then rinsed with PBS buffer at pH6. 8, fixed with Carnoy's fluid, stained for 8min with Giemsa (1 : 20, pH6. 8), and colonies containing more than 50 cells were scored. Their relative biological effectivenesses ( RBE) were investigated. The results show that RBE depends on cellular types and increases with increasing of cellular survival level when LET is at 125.5keV/mum, and decreases with increasing LET when LET greater than or equal to 125.5keV/mum.