The XTR public key system was introduced at Crypto 2000. It is regarded that XTR is suitable for a variety of environments, including low-end smart cards, and XTR is the excellent alternative to either RSA or ECC. In [LV00a,SL01], authors remarked that XTR single exponentiation (XTR-SE) is less susceptible than usual exponentiation routines to environmental attacks such as timing attacks and Differential Power Analysis (DPA). In this paper, however, we investigate the security of side channel attack (SCA) on XTR. This paper shows that XTR-SE is immune against simple power analysis under assumption that the order of the computation of XTR-SE is carefully considered. However, we show that XTR-SE is vulnerable to Data-bit DPA, Address-bit DPA, and doubling attack. Moreover, we propose countermeasures that prevent the proposed attacks. As the proposed countermeasure against doubling attack is very inefficient, a good countermeasure against doubling attack is actually necessary to maintain the advantage of efficiency of XTR.