The purpose of this study was to examine the effect of the type of goals and ability on critical thinking performance. Seventy Three students at a nursing school evaluated 30 items of enthymematic reasoning that is reasoning based on enthymeme under two kinds of goals (“to make correct judgments” and “to enjoy things”). Enthymeme, in this study, was defined as a modus ponens syllogism without an unstated premise, so that the enthymeme was logically invalid. We also manipulated the degree of believability of the unstated premise and classified the 30 items of enthymematic reasoning into four categories based on the degree of believability. Critical thinking ability was measured by using the inference subscale of the Watson-Glaser Critical Thinking Appraisal. The results showed that the logically invalid enthymematic reasoning was more accepted under the goal of enjoying things. Students less accepted the enthymematic reasoning with a less believable implicit assumption. Although students tend to accept the enthymematic reasoning with a relatively believable assumption, the students who had high critical thinking ability more refused to accept the enthymematic reasoning under the goal of making correct judgments in comparison with the students who had low critical thinking ability. We conclude that (1) critical thinking performance varies with goals and beliefs in implicit assumption within subjects; (2) high score in critical thinking ability test does not necessarily guarantee high performance; and (3) critical thinking ability affects performance interacting with goals.