The composite materials chosen for any application, in addition to functional efficiency and improving the various properties of the structures should fulfil some criterion, for the cause of sustainability and a better quality at various environmental conditions. Keeping this in view, the present work has been undertaken to study the water absorption and thickness swelling behaviour of a polymer matrix composite (Unsaturated Polyester) using alkali treated bagasse fibre as reinforcement. Bagasse fibre is alkali treated to improve the interfacial bond strength and adhesion between the matrix and the fibres. Alkali treated bagasse fibres were simply randomly mixed with polyester resin, and composite specimens with fiber contents of 10%, 15% and 20% by weight were fabricated by hand lay-up melding technique. Short term physical properties were investigated by water immersion. Results indicated that the water absorption and thickness swelling increases with increasing of fibre weight fractions and immersion time duration.