Abstract: Measuring distance from images is a common task in engineering task and our daily lives.Distance measurement based on single image which only employs the clues in the image has become a new popular technique.Since circles are common in daily life and process good geometric properties,they has been one of the most important clues for single image measurement.We propose two methods of measuring distance using circles in one single image.Both of them need to compute the projected center of circle and vanishing line of the plane firstly.Then we compute the diameter with the first method,which is parallel to the unknown line segment.After that we can obtain the distance between any two points on the plane based on cross ratio.Under the influence of the location and direction of the line segment to be measured,the errors of the results are obvious diverse in different directions.Instead of diameter the second one compute the exterior of circle.Then the core process of the algorithm is transformed to the distance measurement based on square.Tests on synthetic and real images show that the quality of the second one is better.