Abstract: Poisson's ratio is one of the important basic mechanical property parameters of engineering materials, which is of great significance in structural design, optimization, performance characterization and evaluation. Unlike general materials, the Poisson's ratio of damageable-nonlinear ceramic matrix composites (CMCs) exhibits a complex variation rule with external loadings. For 2D-C/SiC composites, the evolutionary behavior of the main Poisson's ratio is investigated through axial tension and compression experiments, and meanwhile the variation law of the off-axis Poisson's ratio is studied by off-axis tension and compression experiments. The experimental results show that the Poisson's ratio is approximately a positive constant under axial compression. However, under on-axis tension, the secant Poisson's ratio changes from positive to negative with increasing stress, while the damaged Poisson's ratio remains positive but decreases with increasing unloading stress. In the high-stress region, both the secant Poisson's ratio and the damage Poisson's ratio tend to be stabilized. Under off-axis compression, both the off-axis secant Poisson's ratio and the damaged Poisson's ratio increase with the increase of imposed stress, and the larger the off-axis angle, the greater the growth rate and the increased amplitude. For off-axis tension case, the changes in secant Poisson's ratio and the damaged Poisson's ratio are relatively small, but both increase with the increase of the off-axis angle. Theoretical calculation results also show that the tensile and compressive Poisson's ratios of 2D-C/SiC composites have significant differences and exhibit nonlinear evolutionary behaviors, indicating the high complexity of their internal damage as well as the deformation mechanisms.