It is known that small amount of MgO in excess is often added to develop pure perovskite single phase of PMN-based composite, however, extra MgO precipitates in grains and inhibits densification of PMN. In this study PMN-PT-BT (PBT) powder was prepared by a conventional mixed oxide method using $(MgCO_3)_4{cdot}Mg(OH)_2{cdot}5H_{2}O$ instead of MgO. The precursor was heated at $500^{circ}C/1h$ and its surface was modified with MgO sol. This effect was investigated in the aspects of sintering and dielectric properties. Small amount of added MgO sol ($0.5{sim}1.0wt\%$) enhanced sintering substantially below $1000^{circ}C$. The PBT with $0.5wt\%$ MgO sol sintered at $900^{circ}C/2h$ had density of $7.62;g/cm^3$, room temperature dielectric constant of 14800, loss of dielectric constant of $1.1\%$, which were comparable to those of the PBT sintered at $1000^{circ}C/2h$. It was noticeable that the extra MgO precipitated mostly on triple points and grain boundaries and resulted in inhibition of grain growth.