Soil surface rock fragment is considered as an important factor on runoff and soil erosion. However, few studies have been focused on quantitative evaluation of the effect of soil surface rock fragments on runoff components as an integral part of soil erosion process especially in natural conditions. The present study has been conducted to evaluate the effect of soil surface rock fragments on runoff components in field plots under rainfall simulation on a clay-loam soil generally less than 1 meter depth and the slope of 20%. Runoff components including runoff commencement time, volume, coefficient and end time were measured in 1×1 m field plots with 6 wide range of surface rock fragments (2±2, 10±2, 18±2, 26±2, 34±2 and 42±2 % of soil surface cover) with 3 replications under simulated rainfall with constant intensity of 40 mm h^-1. The results showed that increasing surface rock fragments up to a specific soil surface cover (about 34%) led to increase infiltration and runoff commencement time, decrease runoff volume and coefficient and runoff end time. The trend of runoff components has been reversed in the range of 34 to 42% of soil surface rock fragments.