The study examined the impact of concept map and concept test on critical thinking ability of undergraduate students of University of Port Harcourt, Rivers State, Nigeria. A sample of 307 fourth year students from three departments in the university was chosen using simple random, purposive and accidental sampling techniques. These students were assigned to three different groups, two experimental treatment groups and one control group. One group was treated using concept mapping, the second group was treated with concept test while the control group received the conventional method of assessment. To guide the study, two research questions and two null hypotheses were developed. The study adopted a quasi-experimental research design by pretest, post-test non-equivalent control group design. The pre- and post-tests data were collected using an adopted 52 item instrument named Cornell Critical Thinking Test (CCTT) Level Z. It is a 3- option (A-C) multiple-choice test with 7 subscales measuring induction, deduction, value judgment, observation, credibility, assumption and meaning. Using Kuder-Richardson 20 formula (K-R20) their internal consistency estimates were 0.81, 0.76, 0.66, 0.71, 0.70, 072 and 0.68 respectively for the subscales, while internal consistency of 0.74 was obtained for the total scale. Data were analyzed using mean, standard deviation, paired t-test, and analysis of covariance. The results obtained indicated that concept map, concepts test and the conventional assessment method independently had positive impacts on critical thinking ability of the students. However, concept test was the most effective, followed by concept map and lastly the conventional method. On the basis of the findings, it was recommended amongst others that school administrators, curriculum developers and faculties should update their curricula by integrating concept test and concept map assessment methods in their instruction as this can improve critical thinking ability of their students.