Abstract: A geological model is established for loess slopes in Heifangtai in Gansu Province It is based on the direct shear tests and the repeated direct shear tests of Heifangtai loess soils.It also collects the physical character and shear strength of soil,and the height and angle of 21 slopes at Heifangtai.The failure probability of the slopes were calculated by the Monte-Carlo method.The results show that cohesion of loess in Heifangtai is nearly zero,and the probability of the slopes is correlated with the angle of slope.If 5%is acceptable probability,the stable angle of slope is 27.2.The slope with angle more than 27.2needs to be protected as soon as possible.