In this study, we used functional near infrared spectroscopy (fNIRS) imaging to investigate the independent levels of oxygenated, deoxygenated, and total hemoglobin (oxy-Hb, deoxy-Hb, and total-Hb, respectively) at the sensorimotor cortex during hand-grasping motor tasks. Our results showed that the activation of contralateral primary motor cortex (M1) exhibited increased oxy-Hb and reduced dexoy-Hb after hand grasping began. Meanwhile, the contralateral primary somatosensory cortex (S1) was deactivated with reductions of both oxy-Hb and deoxy-Hb concentration. The Hb circulation patterns indicated that the hand grasping demanded rapid and sufficient O2 supply at contralateral M1, which was achieved by the local vasodilation. The contralateral S1 presented decreased total-Hb via the mechanism of vasoconstriction, and maintained the local oxygenation level in a relatively stable state (mostly with O2 debt) to compensate the blood demanding at nearby M1. This study presented that fNIRS data can efficiently differentiate the activation of M1 from the deactivation of S1 during motor tasks, which can provide full interpretations of hemodynamic response to the neuronal activation in comparison with the Blood Oxygenation Level Dependent signal of functional magnetic resonance imaging.