Abstract: This paper gives the method of Helmert-Weight Factor of Variance Component Estimation, and then, the formula, the condition that can be estimated are given too. In the view of mathematical analysis, it is possible that the negative variance exists and why it exists. In the same time, comparing the method of Helmert-WF with that of Helmert, the defect of Helmert method is pointed by a special mode. Given examples show the conclusions are correct.