Carbon micro-clusters are accelerated by an HI-13 tandem accelerator.The plastic nuclear track detectors CR-39are irradiated by C1-C4 beams from the HI-13 tandem accelerator and the tracks in CR-39 are studied using an atomic force microscope(AFM).The depths and diameters of C1-C4 tracks are measured for the first time in a nanometre scale.An enhancement of the energy loss is obtained for carbon clusters related to single carbon ions with the same velocity.The results show that the AFM observation is very useful in the quantitative analysis of clusters in the track detector CR-39.