Lack of valid means of measuring explicit and implicit knowledge in acquisition of second language is a concern issue in investigations of explicit and implicit learning. This paper endeavors to validate the use of four tests (i.e., Untimed Judgment Grammatical Test, UJGT; Test of Metalinguistic Knowledge, TMK; Elicited Oral Imitation Test, EOIT; and Time pressured Judgment Grammatical Test, TJGT) from a set of instruments introduced by Ellis (2005) to measure explicit and implicit knowledge of ESL learners. The result of Principal Component Analysis shows the UJGT and TMK loaded on the first factor (i.e., explicit knowledge) and the EOIT and TJGT loaded on the second factor (i.e., implicit knowledge) when a two factor solution was imposed. The study also shows that second language... students respond in a different way to ungrammatical and grammatical sentences in the UJGT. Hence, Pearson Product Moment Coefficient tests have been conducted amid the ungrammatical and grammatical sentences in the UJGT and other instruments. The outcome suggests that in the case of UJGT ungrammatical sentences would provide a superior measurement of explicit knowledge.