Working memory is one of the most important functions in our brain, which has been widely studied with unreal-life measured technologies. A functional near-infrared spectroscopy (fNIRS) instrument with a portable and low-cost design is developed, which is capable of providing hemodynamic measurement associated with brain function in real-life situations. Using this instrument, we performed working memory studies involved in Chinese words encoding, verbal, and spatial stem recognition, which are mainly studied with other technologies. Our results show that fNIRS can well assess working memory activities, in comparison with the reported results mainly using other methodologies. Furthermore, we find that hemodynamic change in the prefrontal cortex during all working memory tasks is highly associated with subjects’ behavioral data. fNIRS is shown to be a promising alternative to the current methodologies for studying or assessing functional brain activities in natural condition.