In this report, we propose lossless/lossy coding gain as a new objective criterion to theoretically evaluate lossless and lossy coding performance of the lossless/lossy wavelet (LLW). The proposed lossless/lossy coding gain consists of three parameters: "lossless coding gain", "quantization-lossy coding gain" and "rounding errors". Performances of 15 kinds of the LLW are measured with two-dimensional (2D) octave-decomposition. Some standard images are applied as input signals to evaluate coding performance of the LLW.