This objective of this work was to study the effect of physical and chemical pretreatment of water hyacinth by size reduction and black liquor on biogas production. Biogas production was estimated by biochemical methane potential (BMP). The condition for the reactor experiment was selected from the condition with the highest biogas yield from the BMP assays. Water hyacinth was low in lignin, and was high in cellulose and hemicellulose. The ranges of cellulose, hemicellulose and lignin of water hyacinth were in 19.88-20.21%, 32.78-33.91% and 4.22-4.42%, respectively. The results of physical pretreatment by size reduction for 39 days showed that the biogas yield of water hyacinth with the size of less than 5 mm, was the highest at 0.258 L/g VS. The result of chemical pretreatment by black liquor for 40 days showed that the biogas yield of water hyacinth with the size of less than 5 mm, 24-h retention time and 80% black liquor was the highest at 0.124 L/g VS. For the single stage anaerobic experiment, using two 2.5-L reactors and 38-46 day incubation period, the water hyacinth with size reduction had 1.82-1.96 times more biogas yields than those with chemical pretreatment. Because the pH in the chemical pretreatment reactor was not appropriate for methanogens. The percentage of methane was 61.50% and 43.69% for the size reduction reactor and the chemical pretreatment reactor, respectively. Thus, the physical pretreatment of water hyacinth by size reduction was more appropriate than the chemical pretreatment by black liquor for biogas production purpose.