Generally, two star trackers with small FOV (field-of-view) are used to maintain a high accuracy for micro-satellite’s attitude determination. Due to the small FOV, a sufficient number of stars for identification cannot be always captured. Combined images technique can be used to ease this problem, but its efficiency is highly affected by the mounting angle between two star trackers. The role made by the mounting angle is analyzed and simulated here. Results show that the number of guide starpairs matched to the reference angular distance changes like the sine function as the reference angular distance changes, and it reaches its extremum when the angular distance is about 90 degrees. And it increases as the tolerance increase. The extremum difference of the numbers of guide starpairs is highly affected by the tolerance. Therefore, the mounting angle should not be 90 degrees in combined images technique. The efficiency of the identification is higher as the mounting angle is smaller.