Water hyacinth was introduced as an ornamental crop species in many countries more than a century ago, because of their attractive blue, lilac to purplish flowers and round to oval leaves. Soon, it was realized to be most invasive and noxious weed. Due to its fast spread and congested growth, it clogs up rivers, hydroelectric plants, waterways and entire lakes, killing aquatic life, hampering river transport and fisheries, endangering the livelihoods of millions of poor people. Many efforts were made to eradicate this weed but it has successfully resisted all the attempts. When looked from a resource angle, it has high content of fermentable matter which makes it a potential source for biogas production. Hence possibility of converting water hyacinth to biogas has been an area of major interest for researchers. In the present work experiments were performed in 250 ml batch digesters with primary sludge inoculum of different incubation period to understand the impact of incubation period on biomethanation of water hyacinth. All the digesters were operated in mesophilic condition with detention time of 60 days. Results showed inoculums of different incubation periods have remarkable effect on biogas production. In particular the digester with primary sludge inoculum of 30 days incubation period produced 0.44 l/gVS which was 91% more in comparison with the control digester.