AIM:To assess the relationship of trochlear nerve and congenital superior oblique palsy and find out the etiologies of congenital superior oblique palsy，to provide theoritical basis for the clinical dignosis and treatment of congenital superior oblique palsy. METHODS:In 16 patients with congenital superior oblique palsy and 11 normal people, the trochlear nerve was depicted with three-dimensional (3D) Fourier transformation constructive interference in steady state (CISS) 3.0T magnetic resonance imaging (MRI), whereas the adjacent vessels were detected with 3D time-of-flight (TOF) MRI.The identification rates of the two groups were studied to find if there was any abnormal with the trochlear nerve of patients with congenital superior oblique palsy. RESULTS: 3D-CISS MRI depicted the proximal cisternal segment of trochlear nerve in the patient group and normal group with identification rates of 27.2% and 812%.The difference between the two groups was statistically significant(P<0.05). CONCLUSION: The abnormal of trochlear nerve is one of the etiologies of congenital superior oblique palsy.