This paper discusses the error transfer from the slope of optical surface to the focus ray. It presents a general equation to calculate the standard deviation of the refractive ray error from that of slope error of the optical surface through geometric optics analysis, applying the equation to calculate the standard deviation of the focus ray error in 6 kinds of solar concentrator, and providing typical results. The results indicate that the slope errors in two directions are transferred to any one direction of the focus ray when the incidence angle is more than 0; for a point focus Fresnel lens, a point focus parabolic glass mirror, and a line focus parabolic glass mirror, the error transferring coefficient from the optical surface to the focus ray will increase when the rim angle or distance of reflection or refraction point to the axis increases; for a TIR-R concentrator, it will decrease; for a glass heliostat, it relates to the incidence angle and azimuth of the reflecting point. The results show that the slope error of the optical surface may be enlarged more than ten folds to the focus ray to decrease the optical efficiency of the solar concentrator greatly. (C) 2013 Elsevier Ltd. All rights reserved.