In designing the landing gear of an airplane, one of the most important factors to consider, is the effect of landing airplane on body, and the role of shock absorber as an energy absorber resulting from landing. In this thesis, the landing gear of airplane has been analyzed using two approaches namely theoretical and simulation software. First, dynamic equations of landing airplane were investigated in six degrees of freedom and then, these equations were converted into space state and consequently solved using MATLAB software. In second approach, the airplane was modeled in ADAMS and different states of landing such as landing with two wheels, landing with three wheels, passing over a bump and taxi were compared and analyzed. Finally, using the above approaches velocity and displacement of airplane mass center were investigated, discussed and compared.