Abstract: This paper takes siltstone,granite and marble with a diameter of 20~50 mm and a height-diameter ratio of 2︰1 as research objects. It examines the influence of size change and lithology on the stress-strain curve,the failure,the uniaxial compressive strength,the elastic modulus and the Poisson's ratio of the rock samples,and analyzes the causes of size effect. The test results show that with the increase of the size,the coincidence degree of the stress-strain curves of the three rock samples increases gradually. The failure mode of siltstone and marble develops from single inclined plane shear failure to X-shaped conjugate plane shear failure. The granite is mainly the X-shaped conjugate plane shear failure,and the failure degree increases gradually. The uniaxial compressive strength and elastic modulus of the three rock samples increase as a whole,and the increase rate is larger between 20 and 30 mm,and smaller between 30 and 50 mm. The failure modes of different rock samples are roughly the same,but the dispersion of stress-strain curve,the stress-strain curve characteristics after peak,and the degree of damage vary. Poisson's ratio has no definite size effect. The local measurement method of resistance strain gauge can not well reflect the deformation characteristics of rock specimen. The main reason for the size effect is the heterogeneity of rock sample,followed by the end effect.