Sugarcane straw can be used as a renewable energy, for heat and power generation, in replacement of fossil fuels. However, the high contents of ash, chlorine, sulfur and alkali oxides of straw may contribute to slagging, fouling and corrosion in thermal conversion systems and consequently its limited use, the straw is burned in the fields to reduce its accumulation causing pollution. In this study, the effects of water washing-roasting pretreatment to reduce ash, chlorine, sulfur and alkali oxides of sugarcane straw were investigated. Straw was cut to 2-6 mm particle size and washed in 3 liters of distilled water at 80 °C during 30 minutes. Washed samples were sun dried and roasted at 250 °C and 20 minutes residence time. Untreated and washed-roasted straw were characterized by ultimate analysis, proximate analysis, higher heating value and ash chemical composition. Fouling and slagging indexes were determined for prediction of ash deposition. The results indicated that washing-roasting pretreatment significantly improves the thermal behavior of sugarcane straw. Ash, chlorine, sulfur and alkali oxides contents decreased by 18.7%, 98.6%, 57.1% and 50.0% of dry matter respectively while higher heating value increased by 26.4%. Grinding properties of the residue were also improved.