Aiming at the tremendous computation and the low rate of standard ray casting algorithm,a new speed-up algorithm of ray casting volume rendering was presented to realize the real-time manipulation of medical images.Firstly,with the uniform changes characteristics of primitives before and after matrix transformation,all re-sampling points except for the six vertex were deduced so as to reduce matrix computation greatly.Secondly,the boundary box technique was utilized to avoid the sampling in empty voxel.Thirdly,extending the Bresenham algorithm to three-dimension,casting ray voxelizing and the re-sampling parameters were computed simultaneously,which greatly improved the efficiency of ray casting.All results of a mass of contrasting experiments show that the acceleration algorithm can produce the same quality images as the conventional standard algorithm ,but the velocity of volume rendering is increased by two or three times.