In this paper, a monocular vision method is proposed to measure the distance between the robot and the obstacle. Firstly it converts the RGB image which is obtained from the robot to HSV image, then detects and identifies the obstacle in the HSV color space. Secondly it calculates the distance between the robot and obstacle using pinhole imaging principle and geometric coordinate transformation. Finally the method is tested on the Darwin humanoid robot; the result from the experiment proves the effectiveness and feasibility of the method.