We grew high-quality single crystals of NpAl 4 , together with UAl 4 , by the Al-flux method and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization and de Haas–van Alphen (dHvA) effect. NpAl 4 is an antiferromagnet with the Neel temperature T N =49 K. The corresponding magnetic moment is most likely directed along the [010] direction in the orthorhombic crystal structure. The large magnetic entropy at T N and the Curie–Weiss behavior suggest the localized nature of 5 f -electrons with the 5 f 4 configuration. The detected dHvA frequencies in NpAl 4 are small, indicating that the small magnetic Brillouin zone is formed in the antiferromagnetic state. On the other hand, the large Fermi surfaces are observed in a paramagnet UAl 4 . The detected cyclotron mass is relatively large, ranging from 3.4 to 23 m 0 , which is consistent with the electronic specific heat coefficient γ= 89 mJ/(K 2 ·mol).