In this paper, we propose an adaptive 3D-DCT compression technique, which dynamically determines an optimal size of the video cube based on the motion analysis. The technique consists of two steps: (a) it analyses the motion within a small (16x16) video cube of eight successive frames, and (b) it selects the size of the cube based on the motion analysis and applies the 3D-DCT algorithm on the selected video cube. The effectiveness of the proposed technique is illustrated by implementing it to a number of video sequences with low, medium, and high motion. Categories and Subject Descriptors I.4.2 [Image Processing and Computer Vision]: Compression (Coding). General Terms Algorithms, Design, Experimentation. Keywords Video compression, three-dimensional DCT, adaptive block size, motion analysis.