Roundness measurement by mechanical V-block method is simple and widely used in industry. However, work surface to be measured may be scratched by the friction between the cylinder work and V-block support and the roundness evaluation is restricted to one cross section because it is based on the three-point measurement system. The optical V-block method by which perfect non-contact measurement can be achieved is proposed and its measuring principle is described in this paper. The measuring error and measurable wavelength/amplitude are made clear by geometrical consideration and some simulation results are presented. This system is also expandable to the cylindricity evaluation and its implementation is shown.