Abstract: Target radiometric error deteriorates SAR image quality. A method is proposed in this paper to analyze theoretically target radiometric error resulting from SAR Block Adaptive Quantization (BAQ). Meanwhile, thermal noise is taken into consideration. During the analysis, first, the process of BAQ is described by sign function. Second, the output signal from BAQ is expanded into harmonics by Bessel function of first kind. Third, the harmonics are processed by matched filter in frequency domain, and finally the analytical expression of target radiometric error is obtained as expected. Numerical experiments of single point target and multi-point targets with thermal noise are implemented to verify this method, and BAQ compression ratios in the experiments are 8:1, 8:2, 8:3 and 8:4. Analytical results derived in this paper could be quite useful for the decision of BAQ compression ratio and the process of target radiometric correction.