In biomass fuel pelletizing, some raw materials with less favorable binding properties require a binding enhancing additive for enhancing required pellet quality. Starch is commonly used as a binder in industrial fuel pelletizing but is a refined product that adds substantially to pellet production costs. In this study, finely milled cassava stems, a starch-rich, cheap, and underutilized byproduct from food production, were used and compared to refined starch as an additive in biofuel pellet production. The evaluation was performed in an experimental design with the factors cassava stem/starch content, moisture content, and material temperature. Measured responses were pellet bulk density, mechanical durability, amount of fines, pelletizer motor current, coefficient of variance for pelletizer motor current, CVA (a measure of process stability), pellet temperature, die temperature, and pellet moisture. Each response was modeled by multiple linear regression (MLR). Cassava stem addition gave similar effects as starch addition by increasing pellet durability and reducing the amount of fines, particularly at low moisture contents (MC similar to 11%). The highest pellet durability in the study was achieved at a low moisture content (11% MC) when using cassava stem as an additive. Combustion properties of the pellets were determined in a residential pellet burner. Low emissions and no ash fouling were obtained for both of the additive types. In conclusion, cassava stem powder is a good additive substitute for refined starch to increase fuel pellet quality.