Recently, 211At has received increasing attention as a potential radionuclide for cancer radioimmunotherapy. It is a α-particle emitter, which is extremely effective against malignant cells. We demonstrate a method to verify the efficiency of 211At-labeled trastuzumab antibodies (211At-trastuzumab) against HER2 antigens, which has not been determined for radioimmunotherapy. A CR-39 plastic nuclear detector is used for measuring the position and the linear energy transfer (LET) of individual 211At α- particle tracks. The tracks and 211At-trastuzumab-binding cells were co-visualized by using the geometric information recorded on the CR-39. HER2-positive human gastric cancer cells (NCI-N87), labelled with 211At-trastuzumab, were dropped on the centre of the CR-39 plate. Microscope images of the cells and the corresponding α-tracks acquired by position matching were obtained. In addition, 3.5 cm × 3.5 cm macroscopic images of the whole plate were acquired. The distribution of number of α-particles emitted from single cells suggests that 80% of the 211At-trastuzumab-binding cells emitted α-particles. It also indicates that the α-particles may strike the cells several times along their path. The track-averaged LET of the α-particles is evaluated to be 131 keV/μm. These results will enable quantitative evaluation of delivered doses to target cells, and will be useful for the in vitro assessment of 211At-based radioimmunotherapeutic agents.