The helical gear pair is widely used in different mechanical constructions such as vehicle industry and working machine industry. The main property of them is the skew tooth direction with which better tooth connections could be available than in case of straight tooth direction. The analysis of the effect of the addendum modification coefficient for the normal stress and the normal deformation is the aim of the research which could be determined by finite element analysis. Five helical gear types are designed and modelled. The difference among them is only the different addendum modification coefficients. All of the other initial parameters are constant. After the result we determine the conclusions based on the diagram's results which are received by the mechanical parameters and the addendum modification coefficients.