The purpose of the current research was to investigate water/moisture absorption and associated thickness swelling and their rates in hybrid wood flour/E-glass fiber/propylene composite immersed in water, exposed to steam, and the influence of application of the glass fiber as well as maleic-anhydride-polypropylene (MAPP 2%) as coupling agent on those properties. The sample strips with 10 mm thickness and 70 mm width were made by using a laboratory twin-screw extruder. The specimens were immersed in water and exposed to steam for 4320 min to determine the water/moisture absorption and the thickness swelling as well as their rates in the composites. Results revealed that application of the glass fibers increased in the water/moisture absorption significantly in the hybrid composites due to formed micro-gaps in interfaces between wood flour and the glass fibers. However, application of the coupling agent (MAPP 2%) significantly reduced the micro-gaps and diminished the water/moisture absorption as well as the related thickness swelling in the hybrid composites.