By using a self-designed lysimeter to measure surface runoff on a slope in Taihang Mountains,we analyze the impacts of the factors influencing runoff in the Granitic Gneiss Region (GGR) of the mountainous area. The analysis shows that (1) rainfall intensity is a key factor for initiating runoff. Surface runoff occurs when rainfall intensity is above a critical value. The gentler the slope,the higher is the critical value of rainfall intensity needed to initiate runoff. When rainfall intensity reaches a critical value,runoff becomes positively correlated with rainfall significantly. However,the correlation between runoff and rainfall intensity is insignificant. (2) Runoff does not increase with the increase in the catchment area. A strong slope infiltration capacity enhances convergent runoff infiltration into soil. Surface slope is also a key factor influencing the process of runoff in the GGR area. With increased slope,surface runoff increases significantly. Since different slope aspects have different soil textures,runoff on sun-ward slope is higher than that on shade-ward slope. The averaged runoff coefficient for sun-ward slope is higher than that for shade-ward slope as well. Generally,however,runoff coefficient is low in the study area. For instance,it has the maximum of 7.3% for a 2 meter-length slope. An important finding from the study is that there exists a very strong infiltration capacity on the GGR slope in the study area. This has a significant implication for understanding the mechanisms of surface runoff,infiltration,and groundwater recharge in the region.