In this study, we show that the efficiency and carrier life time of multi-crystalline silicon solar cells were significantly improved by using a low pressure (20,000 Pa) and temperature (650 degrees C similar to 750 degrees C) environment grown thermal oxide (TO) as the surface passivation layer. In this experiment, during the first stage, the oxidation process was done at 650 degrees C and a lower pressure of 20,000 Pa for 2 mins under the flow a gas mixture of N-2/O-2 in ratio of 2:1. In the second stage, a temperature of 750 degrees C was used at the same pressure for the post-growth annealing process under a pure N-2 ambient for 25 mins. Consequently, conversion efficiency was significantly increased by 0.55% with the surface passivation layer grown by low pressure and temperature TO process. The sheet resistance, carrier lifetime, internal quantum efficiency (IQE), increased by 6.32 Omega/sq., 22.18 mu s, 4.33%, respectively, and the average reflection was reduced of 0.62%. Thus, the low pressure and temperature thermal oxidation process was an efficient way to increase the efficiency of the multi-crystalline silicon solar cells.