The heating rate, firepower, and thermal efficiency of a modified rocket stove using sawdust and liquid petroleum gas (LPG) as co-firing fuel were investigated. Three modified rocket stoves with a height of 400 mm and outside diameters of 225, 385, and 550 mm were tested. It was found that there was an insignificant difference in heating rate and firepower when stoves were tested without co-firing with LPG. In this case, the stove heating rate was in the range of 1.49−1.55 °C/min. When LPG was used, the heating rate tended to linearly increase with the increase of LPG flow rate. The heating rate was in range of 2.42−2.80, 2.63−3.27, and 3.07−4.22 °C/min when LPG consumption rates were 2.38 × 10−5, 3.33 × 10−5, and 5.00 × 10−5 kg/s, respectively. The slight increase of stove heating rate and firepower was seen when the stove diameter was increased from 225 to 385 mm. The increase of stove diameter from 385 to 550 mm resulted in a huge increase of heating rate and firepower. Thermal efficiency of the sawdust stove without LPG decreased from 17.90% to 9.97% when the stove diameter was increased from 225 to 550 mm. For co-firing of sawdust and LPG, the increase of LPG flow rate from 2.38 × 10−5 to 5 × 10−5 kg/s caused the linear increase of thermal efficiency from 20.27% to 33.80%, 29.36% to 38.89%, and 25.25% to 36.39% for the stove with diameters of 225, 385, and 550 mm., respectively.