This paper presents an experiment of the preparation of ensilage with wet rice straw (WRS) treated with biogas slurry (BGS) and ensiled with molasses to increase the nutritional and preservation quality of straw in monsoon season of Bangladesh in the Department of Animal Science, Bangladesh Agricultural University (BAU), Mymensingh. Chopped wet straw was preserved in plastic containers under airtight condition at room temperature based on the treatments as T 0 (100% WRS only), T 1 (0% BGS), T 2 (5% BGS), T 3 (10% BGS) and T 4 (15% BGS) with 5% molasses as DM basis in each treatment except T 0 to investigate physical quality, chemical composition, in vitro organic matter digestibility (IVOMD) and metabolizable energy (ME) content at 0, 30, 45, 60 and 90 days. The highest CP content was found to be 7.70% in T 4 and highest DM and EE content were found to be 31.42 and 4.86% in T 0, respectively. The lowest CP and DM content were found to be 4.02 and 21.94% in T 0, respectively and lowest EE content was found to be 3.13% in T 4 . The CP and DM were increased (P<0.05) and EE were decreased (P<0.05) with the increasing of ensiling time from 0 to 90 days. The highest OMD and ME content were found to be 48.46% and 6.98 MJ/Kg DM in T 4 and the lowest OMD and ME content were found to be 34.31 and 4.84% in T 0, respectively. The pH value was also decreased (P<0.05) with the increasing of BGS and ensiling time. Considering all the physical and chemical properties, among all the treatments, T 2 and T 3 are acceptable for preparing ensilage. Ensilage of WRS with biogas slurry will not only reduce waste disposal and pollution problem, but also provide inexpensive feed components for ruminants.