The superficial degradation in vivo and biocompatibility of magnesium alloy (Mg-Al-Zn) coated with tricalcium phosphate (β-TCP) have been evaluated. (β-TCP)-Mg-Al-Zn implants were made and implanted into the femur of rats. The surface microstructure observation of implants and the EDS analysis were carried out. The changes of Ca and P on the surface of the implants were analyzed. Results show that many attachment proteins appear in the surface of (β-TCP)-Mg-Al-Zn after 4 weeks implantation. In the 8th week, the content of Ca and P in the surface of (β-TCP)-Mg-Al-Zn are higher than that in naked Mg-Al-Zn (P＜0.05). In the 12th week, about 33 % of naked Mg-Al-Zn alloy is degraded, while 17 % of (β-TCP)-Mg-Al-Zn is degraded. β-TCP coating can decay the superficial degradation and improve the biocompatibility of Mg-Al-Zn matrix in vivo.