Determining what to explain and generating explanations have to be satisfied to do successful self-explanations. Providing prompts helps the first part, and students having high Need for Cognition can help the whole process. Seventy four adult vocational school students participated in a three factors (ie., self-explanation, prompt, Need for cognition) between subjects experiment. The results were different depending on the tasks and the students level of Need for Cognition. In memory tests, asking to do self-explanations or giving prompts helps students of low levels of Need for Cognition. However, asking to do self-explanations or giving prompts was not effective to students of high level of Need for Cognition. In tests measuring understanding, the main effect of Need for Cognition was significant, and three factors interaction effects was marginally significant. Giving prompts helped students of high Need for Cognition understand better. The results were interpreted in terms of cognitive load.