A new orthogonal transform coding scheme by Zernike polynomials for CT image archiving is discussed in this paper. By considering the image formation process, it can be shown that all of the meaningful information about object is contained in the set of lower order transform coefficients, which maximum order is determined by the measuring condition of projection data. Thus, the practical CT images can be exactly represented by a finite number of transform coefficients. In the proposed coding scheme, these coefficients are extracted and quantized according to the bit allocation rule which is prescribed by the statistical nature of CT images. Since both structual and statistical redundancies are reduced, an information lossless reversible data compression with a high compression ratio can be achieved. The experiments were carried out to confirm the coding characteristics.