Functional near infrared spectroscopy (fNIRS) is a non-invasive technology that can be applied in brain-computer interface (BCI). INIRS uses light in the range of near infrared to measure activity of the cerebral cortex. Prefrontal cortex is activated when performing mental arithmetic tasks. We capture frontal hemodynamic response during mental arithmetic task from two subject. Two different brain activities, rest and active, are recognized using Support Vector Machine (SVM). Task onset delay can be reduced by selecting appropriate feature set.