This research aims to know the effect of Project Based Learning (PjBL) based on macroalgae potency of seashore toward students critical thinking skills. This research was held at SMAN 1 Tanjungsari in academic year 2013/2014. All student of grade X MIA had been chosen as a research population. Randomly, two classes were selected representing the experimental class (implementation PjBL based on macroalgae potency of seashore) and the control class. A quasi-experimental method was designed to achieve the research objectives. Data were collected during one month or four meetings consist two hours (45 min each). Critical thinking was measured through essay test which were formulated based on cognitive scale of Bloom taxonomy (C4–C6). All the data were analyzed using t-test method with α=5%. Observation of learning process was also done to get information on critical thinking process. The result showed that implementation of PjBL based on localities influenced significantly on students' critical thinking skills.