A combined process of a mechanical ball milling and liquid glycerin ($C_{3}H_{8}O_3$) treatment of boron (B) powder has been conducted to enhance the superconducting properties of $MgB_2$. The individual aims of the mechanical milling and the glycerin treatment were to reduce the grain size of the $MgB_2$ and to achieve homogeneous carbon (C) incorporation into the $MgB_2$, respectively. Four kinds of B powders of as-received, glycerin treated, 2 h milled, and 2 h milled + glycerin treated were prepared. $MgB_2$ bulks were fabricated by in situ process using the prepared B powders. The mechanical ball milling was effective for a grain refinement, and a lattice disorder was easily achieved by glycerin addition. It was found that the critical current density ($J_c$) values were enhanced in the samples with milled B or glycerin treated B only. In the $MgB_2$ bulk prepared with both milled and glycerin treated B, the $J_c$ was further increased due to a higher grain boundary density and a greater C substitution.