Water hyacinth (Eichhornia crassipes) is a noxious weed that has attracted worldwide attention due to its fast spread and congested growth, leading to serious problems in fishing, evapotranspiration, navigation, irrigation, power generation, and reduction in biodiversity. However it is a potential source of biomass to produce ecofriendly biogas. In this study, primary sludge from sewage treatment plant was used as inoculum to increase biogas production from biomethanation of water hyacinth at mesophilic condition. Series of laboratory experiments using 250 ml biodigesters were performed in batch operation mode. 4 grams of completely dried and ground water hyacinth were fed to each biodigester and mixed with primary sludge inoculums (PSI) and water in different combinations resulting in five different fermentation slurries (PSI-0, PSI-25, PSI-50, PSI-75 and PSI-100) with different total solids of 3.8, 5.5, 7.2, 8.9 and 10.6% respectively. The results showed that the PSI inoculated to biodigester improved biogas yield significantly and was almost two times, compared to Water Hyacinth substrate without PSI. The best performance for biogas production was from the digester PSI-75 followed by PSI-50 and PSI- 100 whose TS contents are 8.9, 7.2 and 10.6% respectively. These results suggest that, TS content affects the biogas yield and optimum total solids content for biogas production is between 7 and 9 %.