Abstract: An accelerated ray casting algorithm for volume data is presented in this paper.The problem for sampling along a 3D ray can be viewed as the problem of 3D scan conversion of a ray. Random access of 3D volume data is avoided by using layer as the basic processing unit. Fast 3D scan conversion of ray is achieved by using look-up table based on the fact that the ray has the same direction in parallel projection. It is shown that the algorithm is better on memory requirement and speed than traditional algorithm through its realization on PC.