Along with the increase of demanding mental effort work, sleep loss, extended work period, mental fatigue is a very common phenomenon in everyday modern life. Recently, mental fatigue has been receiving increasing attention from military, transport, health. However, very little is known about the neurobiological mechanisms underlying mental fatigue. The primary purpose of this study was to investigate neuronal activation pattern for circadian mental fatigue. An auditory discrimination task was administered to ten subjects in a 25 hours sleepless experiment. Functional MRI images were acquired at four time points (9 am, 2 pm, 3 am, 9 am the following day) to determine changes in subjects' brain activation. Results showed that the circadian fatigue causes general decreased activity of the brain. Some regions significantly related to mental fatigue included: right superior temporal gyrus, left thalamus, right inferior frontal and middle frontal decreased activity with time, while activation in inferior parietal cortex was found to closely link to circadian rhythm. The results suggest that circadian mental fatigue effects neuronal activation in a particular trend.