Among many dimensional and dimensionless amplitude parameters, Kurtosis (4-th normalized moment of probability density function) is recognized to be the sensitive good parameter for machine diagnosis. Kurtosis has a value of 3.0 under normal condition and the value generally goes up as the deterioration proceeds. But there are cases that kurtosis value goes up and then goes down when damages increase as time passes. In this paper, a simplified calculation method of kurtosis is introduced for the analysis of impact vibration including affiliated impact vibration. Affiliated impact vibration is approximated by triangle and simplified calculation method is introduced. By varying the shape of triangle, various models are examined and above phenomenon is traced and its reason is clarified by the analysis. As peak value grows up, Kurtosis increases, then after the damage spread to other rolling elements, the width of the signal shape spread and Kurtosis falls. When the peak value arises after that, Kurtosis rises up again. Such movement can be confirmed. Many similar cases could be observed. Utilizing this method, the behavior of kurtosis is forecasted and analyzed while watching machine condition and correct diagnosis is executed.