Ellis (2004) had given a specific working definition of explicit knowledge in the definition and measure-ment of L2 explicit knowledge. He said this kind of knowledge is“the conscious awareness of what a lan-guage or language in general consists of and / or of the roles that it plays in human life.” To be more specific, explicit knowledge is part of declarative memory, and it is stored in the limbic system of the brain. Besides, it is always presented in consciousness and can be verbalized. For example, a L2 learner can point out that in this sentence“The boy plays basketball”.“-S”should be put after the predicate“play”, because the subject of the sentence is the third person singular pronoun. Also, the process of learning explicit knowledge is controlled or monitored. In general, experts test explicit knowledge mainly by grammar. But there are also some other aspects of explicit knowledge that need to be considered, such as the pronunciation, vocabulary, pragmatic features and so on. This essay considers what effect of the as-pects I have mentioned above might have on second language learning.