To investigate the feasibility of mental workload assessment by using functional near-infrared spectroscopy (FNIRS), 12 college students are orgainzed to complete N-back and multi-attribute task battery (MATB) tasks with physiological parameter, behavioral performance and subjective scale. The sensitivity and difference of FNIRS parameters of different difficulties are analyzed. FNIRS data of the first, fifth, ninth and tenth channels in dorsolateral prefrontal cortex (DLPFC) area in N-back task and the first, ninth channels in DLPFC area during MATB task are more sensitive to mental workload. In the tasks of 0~2 level difficulty, the oxygenated hemoglobin (HbO) and total hemoglobin (tHb) show greater change as the difficulty increases and lower value change in 3 level difficulty. The results show that the FNIRS signal in DLPFC is the most sensitive area of mental workload in prefrontal cortex (PFC) area. Within a certain range of difficulty, HbO and tHb show larger change as task difficulty level increases, but the change will be opposite when task difficulty beyond a certain level.