Online review has been found to be import to online businesses. The sheer number of online reviews hinders websites users' ability to draw useful information from previous users. Thus, a helpfulness mechanism which facilitates users to locate the most helpful reviews adds value to websites. This paper explores the helpfulness of online review from a cognitive science perspective. Drawing upon cognitive load theory and source credibility heuristics cues, the paper develops a framework to understand the cognitive process of review readers to evaluate the helpfulness of a review. The study testes the proposed model using 2,371 actually reviews from a popular website. The results support the theoretical model. The distinct finding that is different from previous studies is an inverse "U-shaped" effect of word counts on review helpfulness. In addition, the study found an inverse "U-shaped" effect of readability on review helpfulness.