►Hydrothermal pretreatment offer accelerated pre-hydrolysis of rice straw biomass. ► 5% NaOH addition is a mandatory requirement in rice straw hydrothermal substrate. ► Methane production was increased by 222.0% for hydrothermal pretreated substrates. ► Hydrothermal pretreatment found as the promising method for improved biomethanation. This paper presents the results of an experimental batch methane fermentation (at 37°C mesophilic temperature) study carried out on untreated and pretreated substrates of rice straw using NaOH and hydrothermal pretreatments. 3% NaOH pretreatment was given to ground rice straw biomass for 120h at 37°C and hydrothermal pretreatment was given for 10min at 200°C. It was observed that NaOH addition is a mandatory requirement for maintaining a suitable range of pH and starting the biogas production from hydrothermal pretreated biomass slurry of rice straw. The fed substrate concentrations were maintained at 5% TS (50g TS/L). The study revealed into 140.0L/kg VSa biogas and 59.8L/kg VSa methane from untreated rice straw substrate. However, NaOH pretreated substrate resulted into 184.8L/kg VSa biogas and 74.1L/kg VSa methane. Hydrothermal pretreated followed by 5% NaOH added substrate resulted into highest biogas and methane production yields as 315.9L/kg VSa and 132.7L/kg VSa, respectively. NaOH pretreated substrate showed an increase of 132.0% in biogas production and 123.9% in methane production relative to the untreated substrate. However, the hydrothermal pretreated substrate had resulted into an increase of 225.6% in biogas production and 222.0% in methane production relative to untreated rice straw substrate. Hydrothermal pretreatment provided an accelerated pre-hydrolysis of biomass contents during the treatment process and thereby resulted into enhanced biogas and methane production yields.