This paper investigated the effects of cognitive and motor dual tasks on gait performance and brain activities in stroke; 23 stroke subjects performed single walking (SW), walking while performing cognitive task (WCT), and walking while performing motor task (WMT) at self-selected speed. The gait performance was recorded, including speed, cadence, stride time, stride length, and dual task cost (DTC). Brain activities in prefrontal cortex, premotor cortex (PMC), and supplementary motor areas (SMAs) were measured by functional near-infrared spectroscopy during walking. Results showed significant decrease in speed, cadence, and stride length, and increase in stride time was noted in both WCT and WMT compared with SW condition. There was no significant difference in DTC between WCT and WMT. The non-lesioned SMA and most channels of bilateral PMCs exhibited significant increases in the index of hemoglobin differential during WCT and WMT compared with SW. Moreover, gait performance was negatively correlated with bilateral PMCs and lesioned SMA during different walking tasks. In conclusion, deteriorated gait performance was noted in stroke attempting dual tasks. There is no significant difference between the two dual tasks on gait performance. Nevertheless, SMA and especially PMC were crucial in cognitive and motor dual task walking after stroke.