Parts precision is one of the key points in industrial application of rapid prototyping technology (RPT), and also the pivot of RPT research, but presently high precision is hard to achieve. This paper focuses the emphases on the principle of error analysis. Obtain chord errors corresponding to the number of triangles with pro/E software during the STL, and the formula for computing stair errors taking the quarter of sphere for an example during the slicing. In the end, propose the measures to improve the principled errors, viz. sloping slicing, adaptive slicing, direct slicing and so on, and further propose the theoretical range of adaptive slicing thickness.