Legal rationality is some awareness, understanding of rational and realization of the activity which is on the legal basis.In other words, this means that the launching of lawis justifiable. The rationality of the law can be divided into the formal rationality(known as rationality of tool)and the substantial rationality (known as rationality of value). Acting as a concrete manifestation of law ,the penalty is no exception. It has two forms. They are the penalty purpose of formal rationality and the penalty purpose of substantial rationality. And they are indispensable, So, today the penal code all over the world will be showing a trends to combine the two. Undoubtedly, the formal rationality and the substantial rationality of the penalty purpose are the expectations of China.