Abstract: This paper presents our improvement to the Tsai's monoview calibration approach with enhanced ability to principle point issues and higher calibration precision is achieved as well.Taking one grid plane as reference object,well-known radial alignment constrain(RAC)is extended to handle with principle point position parameters firstly,though function-dependency between principle point and other parameters are introduced.Next,vanishing points intersected with the perspective projection of grid lines,parallel to world coordinate system X,Y axis respectively,are related to dependent parameters involved in calibration equation from extended RAC and as a result,new non-linear calibration equation with independent parameters is deprived.After that,procedure using improved approach to calibrate camera parameters is specifically listed.In this step,one Least Square based iterative solution is proposed to solve the non-linear calibration equation as well as initial parameter values are given.Finally,stimulated images generated with OpenGL toolkit with known camera parameters and real image from CCD camera Nikon P5100 are used for calibration experiments.Valuable conclusions are conducted.