YbPt5B2 and LuPt5B2 were studied by elastic and inelastic neutron scattering experiments. The heavy fermion system YbPt5B2 orders antiferromagnetically below T_N1 = 7.8 K and exhibits an incommensurate magnetic structure with a temperature dependent propagation vector, k_1 = (0.194, 0, -0.045) at T = 6 K. Below T_N2 = 4.7 K, a transition into a commensurate antiferromagnetic structure, k_2 = (0, 0, 0), takes place. Rather large Yb moments, m(Yb) > 3 μB can be explained only by an appropriate wave function constituting the CEF ground state doublet as concluded from the present inelastic neutron data. The overall CEF splitting is of the order of 44 meV, while the first excited level is located near 25 meV above the ground state.