Gear transmission is one of the most widely used form in mechanical transmission system and plays an important role in Automobile industry. Manufacturing errors, assembly looseness, gear wear issues in gear transmission system may result in backlash of gear, noise and fatigue damage, which seriously affect the efficiency and service life of gear transmission. For the gear transmission assembled, it is of great importance to obtain the desired results monitoring the condition of gear meshing and prevent the occurrence of dangerous situations. How to determine whether there are some issues of gear tooth wear, misaligned bearing, gear eccentricity and backlash and how to define gear sets with problems and find the specific problems which already exist on gear transmission are of great value and significant to ensure the quality and design of product. In this paper, firstly, gearing mechanism was analyzed, then the manifestations which a series of issues consisting of gear tooth wear, misaligned bearing, gear eccentricity and backlash reflected on gear mesh frequency were summarized. The results showed that wear was indicated by excitation of natural frequencies along with sidebands of 1×rpm of the bad gear, gear misalignment almost always excited second order or higher harmonics with sidebands of running speed and fairly high amplitude sidebands around GMF suggest eccentricity, backlash or non-parallel shafts. Lastly, the methods of gear meshing and order tracking analysis were applied in a transfer case. The results revealed that there existed two problems:one was shaft looseness, the other was misalignment of the planetary gear set. At high gear range, the premier noise sources were related to the input and out shafts looseness. At low gear range, the planetary gear set with eccentricity had shown significant contribution to the noise.