Star tracker is one of the most important devices used on satellites for attitude determination. Since its output is discontinuous, it needs a complementary unit to cover its discontinuities. Using gyroscope unit is the most suitable choice for aiding the Star tracker. However, using these two kinds of sensor simultaneously has some challenges. In other words, not only not only sensor biases decrease the accuracy of attitude determination, but also the installation error has a significant effect on the accuracy. In this paper, after presenting the important role of installation errors between Star tracker and gyroscope in the accuracy of attitude determination, an effective method is proposed to determine the misalignment error between these two sensors which is only based on their measurements, and the mathematical formulation is presented in detail. Finally, to validate the performance of the proposed method, it is implemented to calculate the instantiation error of an experimental dataset gathered in the Mount Pooladkaf, And the results are reported in the form of graphs and tables.