A series of the nano-sized Mn polycrystalline-substituted MgB2 samples were synthesized by solid-state reaction method with the ratio of Mg1-xMnxB2 (x = 0.00, 0.01, 0.03 ,and 0.05). Mn substitution emerges shifting (002) peak position to higher angles on MgB2 samples. This shows that isovalent Mn2+ is substituted by Mg2+ ions. Randomly oriented grains were obtained in surface morphology. The presence of Mn nanoparticles was observed with energy-dispersive spectrometry analysis. The superconducting transition temperature was significantly suppressed by Mn ions, from 39.28 to 26.60 K. Spin-flip scattering and pair breaking effect arose from the magnetic nature of Mn. Increasing Mn content ratio in the samples caused narrower magnetization versus applied magnetic field loops, without magnetic flux jumping. Self-field critical current density decreased from 260 to 16 kA cm(-2) with rising Mn substitution ratio. Scaling behavior of the samples was examined between reduced force (f = F/F-max) and reduced applied field (h = H/H-max). Fitting lines showed that pure MgB2 was dominated by normal point pinning mechanism at both lower and higher magnetic fields. However, Mn-substituted MgB2 samples exhibited surface pinning mechanism due to grain refinement.