A method to measure object distance by monocular vision was proposed.Position of one object′s conjugate image plane varied when the distance between it and the optical system was changed.The image on CCD was out of focus as a result of fixed CCD in cameras.Blur extent of images was scaled by width of image edge.Images were processed by wavelet transform and widths of blurred image edge were detected.Real distance between object and optical system was calculated through cubic spline interpolation.The measurement method to object distance by monocular vision was realized.Results show that the method is feasible.