Thickness optimization of heavily doped p-type seeding layer was studied to improve performance of thin film silicon solar cell. We used liquid phase epitaxy (LPE) to grow active layer of $25{mu}m$ thickness on $p^+$ seeding layer. The cells with $p^+$ seeding layer of $10{mu}m;to;50{mu}m$ thickness were fabricated. The highest efficiency of a cell is 12.95%, with $V_{oc}=633mV,;J_{sc}=26.5mA/cm^2$, FF = 77.15%. The $p^+$ seeding layer of the cell is $20{mu}m$ thick. As thicker seeding layer than $20{mu}m$, the performance of the cell was degraded. The results demonstrate that the part of the recombination current is due to the heavily doped seeding layer. Thickness of heavily doped p-type seeding layer was optimized to $20{mu}m$. The performance of solar cell is expected to improve with the incorporation of light trapping as texturing and AR coating.