Abstract: At the initial stage of rock bearing load, a great amount of microcracks appeared in the material due to microdefects random nucleation and limited growth. In the paper, the mechanism of microcracks evolution at the initial stage rock damage was generalized that the isolated nucleation of microdefects formed the initial microcracks of the smallest size, and the clustering nucleation of microdefects close next to the initial microcracks formed the large-scale microcracks, in which the microcracks size growth was the function of the number of microdefects nucleation. By using the microcracks size and frequency distribution fractal and the microcrack rough surface fractal, two models were established. One was a relationship formula between accumulated number of microdefects nucleation and microcracks size, which was called the Crack Growth Model (CGM). The other was an isotropic damage model based on microdefects nucleation number series, which was called the Damage Evolution Model (DEM). The statistics analysis on the microcracks evolution in a gradually fractured rock plate shows that the predictive cracks growth and microcracks damage evolution are in good agreement with the measured values. Since a microdefect nucleation being accompanied by a acoustic emission (AE) and the number series of microdefects nucleations was similar to the AE series, the two models could be used to predict microcrack growth and microdamage evolution by the use of AE series. It is important to study microcracks growth and damage to the macro mechanical properties for the understanding of the whole course of evolution and prediction of catastrophe.