Arachnoid cysts represent only 1% in all intracranial masses. The signal and density of arachnoid cysts are mostly the same as cerebrospinal fluid unless be complicated. They do not have calcification and do not retain contrast material but they are benign lesions. Although they can be seen in all ages, they are mostly detected in children. Male-female ratio is In this case report., 18 months-old-boy with subfalcine herniation because of an arachnoid cyst who was brought to our hospital only with complaint of failure to thrive was presented due to its atypical and rare presentation.