It has long been recognized that the strength of brittle rocks decreases with the grain size. However, very few systematic investigation of this phenomenon has been made using numerical method. This paper presents the results of a numerical simulation using the Rock Failure Process Analysis code (RFPA2D) to investigate the effects of grain size on the uniaxial compressive strength and the failure behavior of Yuen Long marble. The Weibull distribution with two parameters (m that characterizes the strength heterogeneity, and σ0 that corresponds to the mean strength of an element) selected based on micromechanical basis is used in the RFPA2D code for simulation. The simulated stress-strain curves of Yuen Long marbles with different grain sizes under uniaxial compressive condition agrees well with the experimental study. The progressive failure process was captured in the numerical simulations. Our simulations also reproduced the influence of grain size, with strength scaling approximately with the inverse square root of grain size, which is in agreement with the previous experimental study.