The magnetization of Mn-intercalated layered MnNb3S6 single crystal has been systematically investigated. The angle-dependent magnetization (M(T), M(H), and M(phi)) displays that the magnetization of MnNb(3)S(6 )with H perpendicular to c is much stronger than that with H//c, but no magnetic anisotropy within the ab-plane. In addition, a field-induced magnetic phase transition is found when H perpendicular to c. The investigation of the critical behavior gives the critical exponents # = 0.3681(2), y = 1.3917(1), and 6 = 4.7805(7) with T-c = 44.5(5) K, which belong to the universality class of three-dimensional Heisenberg model. The critical behavior and exponents indicate that the magnetic coupling in MnNb 3 S 6 is of an isotropic short-range type. Based on universality scaling, the H - T phase diagram with H perpendicular to c is constructed. The magnetic behaviors and phase diagram of MnNb3 S6 are analogous to those of CrNb(3)S(6)( )which exhibits a chiral magnetic soliton lattice.