We have investigated various characteristic properties depending on sintering conditions of MgB2 samples prepared by the standard solid state reaction method. It is inferred from experimental results that the crystallinity of samples were improved when the pressure of the Ar ambient increased. Also, it was found that the sintering temperature above 850 degrees C caused extremely high amount of decomposition of the superconductor phase. Finally, it was considered that the sintering process of MgB2 must be carried out under the pressure of Ar ambient higher than 8 bar to impede the volatility of Mg in the structure of MgB2. The J(c) values of samples systematically enhanced with the increase of sintering time and in particular, the sample sintered for 180 min. exhibited the highest J(c) (0) of 4.9 x 10(3) A cm(-2) at 30 K. The obtained results demonstrate that the sintering conditions of MgB2 have a significant influence on T-c ((onset)) and J(c), which are directly related to practical applications of MgB2 based superconductor components.