Abstract: In early researches on velocity speed-up effects, relative simple formulations are often used to calculate the speed-up ratio on crest, but other complex situation is neglected. In order to discuss the influence of the slope and height of hills as well as terrain conditions on speed-up effects, the speed-up ratio has been calculated by Fluent numerical simulation. The results show that the maximum speed-up ratio increases with the increasing of the slope and height of hills and with the decreasing of exponent of ground roughness. Besides, the slope, height and terrain conditions have notable influence on the distribution of the speed-up ratio on crest. Modified guidelines are suggested to correct the Original Guidelines.