There are few researches that consider both cognitive performance and mental workload in the field of intellectual productivity studies and cognitive load theory (CLT) studies, and most of them focus on only one of them. The authors therefore have proposed a Performance-Mental Effort model which deals with both of them. In the model, the cognitive states are classified into four categories which reflect their performance and mental effort. A subject experiment was conducted where their cognitive task performances and physiological indices such as heart rate and pupil diameter were measured. As the result, the estimated states from the measured data based on the proposed model could well explain the detail changes of their cognitive states.