In his Essay, John Locke contends, "since the mind, in all its thoughts and reasonings, hath no other immediate object but its own ideas, … out knowledge is only conversant about them." But it is inconsistent with his realism, since the object of human knowledge is the external world itself rather than the idea in the mind. Thus, how can he be sure that he has knowledge of the world? In this paper, the author deals with Locke`s concept of knowledge by an investigation of the problem in ideas which is related to criteria of truth. The paper is divded into two parts. The first part deals with Locke`s metaphysics. The author contends that Locke develops his theory of knowledge in order to justify his religious faith and metaphysics. Locke is a realist. He establishes his theory of knowledge by way of way of idea. But he does not trust the ability of senses. Mind cannot know the real essence of the material substance. Thus, how can he claim that he knows things, since his theory of truth is of correspondence rather than of cohence? The second part of the paper clarifies Locke`s problem of certainty in terms of origin, nature and division of idea. The author contends that Locke holds a causal theory of perception, since he uses the correspondence theory of truth. But It does not help Locke to get rid of the challenge of scepticism in natural science. In mathematics and morals, Locke believes that the human knowledge can be certain.