Using the basic tools of microeconomics, the article investigates the resources allocation efficiency of patent system, and points out that patent system is aimed to provide rewards to inventors by creating market monopolizing power which necessitates some welfare loss. Despite that patent system isnt the best way of resources allocation, one can construct optimal combination of patent length and breadth according to different conditions, so as to enhance the efficiency of patent system, which has an important implication for the design of patent system.