The objectives of this study were to determine the effect of pretreatment to the garden wastes under thermal and alkali conditions and to evaluate biochemical methane potential. In thermal pretreatment different temperatures and application times were examined and solubilisation improved by heating. Soluble chemical oxygen demand (sCOD) concentration upgraded by increasing temperature with different time intervals. 90 degrees C and 2-hour pretreatment time enabled maximum solubilisation of COD for garden waste. For carbohydrate solubilisation, 50 degrees C and 8-hour application time showed the best performance. Alkali pretreatment also stimulated solubilisation of COD, carbohydrates and total nitrogen which concluded with biochemical methane potential tests. Alkali pretreatment (1% NaOH) increased the overall biomethane production by about 30% compared to the garden wastes without pretreatment. According to this study, the highest methane yield (397.7 ml/gVS) was obtained with 1% NaOH pretreatment to garden waste without pH adjustment.