We have observed the reduction of group velocity of light via electromagnetically induced transparency (EIT) and measured the dependence of pulse delay on the one-photon detuning in a three-level Λ-type atomic system. The results show that due to the effect of Doppler broadening the light speed reduction is signif icant in a range of ±600MHz one-photon detuning. Under the condition of consid ering the Doppler shift of frequency, the pulse delay as a function of the one-p hoton detuning is numerically calculated. The experimental measurements are in g ood agreement with the theoretical results. These researches provide theoretical and experimental references for controlling the group velocity of light by mean s of one-photon detuning.