Twin boundaries were determined in both R-phase and martensite of NiTi shape memory alloy using group decomposition theory. It was found that there are four variants existing in the R-phase with {110}p and {100}p twin planes between the variants. The four variants may form three types of self-accommodation groups. While in the martensite phase, there may be three variants with {110}p twin planes between the variants. The three variants have four possibilities to form a self-accommodation group. These results are in good agreement with the experiment.