This study explored the phenomenon of testing effect during science concept assessments, including the mechanism behind it and its impact upon a learner's conceptual understanding. The participants consisted of 208 high school students, in either the 11th or 12th grade. Three types of tests (traditional multiple-choice test, correct concept test, and incorrect concept test) related to the greenhouse effect and global warming were developed to explore the mechanisms underlining the test effect. Interview data analyzed by means of the flow-map method were used to examine the two-week post-test consequences of taking one of these three tests. The results indicated: (1) Traditional tests can affect participants' long-term memory, both positively and negatively; in addition, when students ponder repeatedly and think harder about highly distracting choices during a test, they may gradually develop new conceptions; (2) Students develop more correct conceptions when more true descriptions are provided on the tests; on the other hand, students develop more misconceptions while completing tests in which more false descriptions of choices are provided. Finally, the results of this study revealed a noteworthy phenomenon that tests, if employed appropriately, may be also an effective instrument for assisting students' conceptual understanding. (Contains 3 tables and 3 figures.)