NpSn 3 is an antiferromagnet below TN = 9.5 K with a low value of the ordered magnetic moment. From the values of the specific heat coefficient in the paramagnetic and ordered phases it can be presented as a moderate heavy fermion. Our results of magnetization and Mössbauer effect confirm previous studies. Furthermore we present resistivity and single crystal neutron diffraction experiments. The latter definitively confirms the type I antiferromagnetic structure which was up to now only suggested by Mössbauer spectroscopy. Results are discussed from both the band and the local 5f points of view.