In this paper, we proposed the compensation method to evaluate arousal level in different initial arousal states. Arousal level was measured by the relationship between IRI and Nz. Since Nz is affected by BI which is directly proportional to initial arousal state of subjects, the arousal level is underestimated To overcome this problem, we proposed the compensation method using modified Karolinska sleepiness scale, and determined compensation coefficients derived from this scale with five arousal levels. Applying these coefficients to portable arousal monitoring system, the proposed method in this paper could be useful for real-time evaluation and control of arousal level. As a result the developed system can detect and control the arousal state from initial drowsing sate.