With modified mutual information(MI) measure,Powell search algorithm is utilized to realize mono-modality image registration.The MI's formulae are improved to reduce computation cost,and the replace of Powell's search direction is also rectified to keep the original search direction and eliminate local maxima.With rotation set in advance,local maxima can be avoided.The influence on registration velocity by Powell convergence threshold is also discussed.The results show that the modified MI measure can reach sub-pixel precision with obviously accelerated speed.It has good robustness,high speed and high precision.