Abstract Composite boards of sugar cane bagasse particles and recycled high-density polyethylene were manufactured by means of a flat press process under laboratory conditions using a partial factorial experimental design (2K−1) to determine the effects of the process variables press temperature, pressing time, bagasse/plastic content and pressure on bending properties, water absorption and thickness swelling. The sugar cane bagasse particles varied in size from 2 to 15 mm, the plastic particles from 1 to 3 mm. Nominal board dimensions and density were 300 mm × 300 mm × 5 mm and 0.75 g/cm3, respectively. The following process conditions were implemented: press temperature 140 and 160 °C, pressing time 6 and 12 min, bagasse/plastic content 40/60% and 60/40%, and pressure 3 and 6 MPa. Bending strength (MOR) and bending modulus of elasticity (MOE) increased proportionally with press temperature and pressing time. An increase of the bagasse content had a positive effect only on bending MOE. An increase in pressure had a negative effect on both bending strength and MOE. Water absorption and thickness swelling were reduced significantly by an increase in temperature and pressing time, whereas variation of pressure did not have any influence on these board properties.