In this study, rice straw (RS) which was one of the major agricultural residues was used as filler in bio‐based thermoplastic polyurethane (TPU). Several treatments including hot water, alkali, permanganate, peroxide and silane were applied for modifying the surface of RS. The effects of surface modifications on mechanical, flow, morphological, and water absorption properties of green composites were investigated at a constant 30 wt% RS loading. Silane treatment performed after alkaline treatment gave the highest tensile strength, elastic and storage modulus in mechanical tests and gave the lowest water uptake properties. POLYM. COMPOS., 37:1596–1602, 2016. © 2014 Society of Plastics Engineers