Carbon fiber/polyethylene (PE) fiber hybrid composites were fabricated by open leaky mold method. The positional effect of PE fiber was investigated concerning the mechanical performance improvement of carbon fiber/PE fiber hybrid composites. The influence of adhesion level of PE fiber on the hybrid properties was studied using oxygen plasma, gamma-MPS, and gamma-MPS-modified polybutadiene (PB/gamma-MPS) as surface modifiers. In case of carbon fiber/vinylester composite, gamma-MPS and PB/gamma-MPS acted as the bridge to bond carbon fiber to vinylester resin through the chemical bonding. In case of PE fiber/vinylester composite, plasma treatment of PE fiber introduced the etching and micro-pitting rather than the functional group on fiber surface. Therefore, the plasma treated composite exhibited a large increase in flexural strength compared with untreated composite. In the case of carbon fiber/PE fiber hybrid composites, the mechanical properties of hybrid composite strongly depended on the reinforcing fiber position. When carbon fiber was at the outermost layer, the hybrid composite exhibited the highest flexural strength among other hybrid composites. This was attributed to the fact that the compressive and tensile stress had maximum magnitudes in the outermost layer. The surface treatment of PE fiber at outermost layer had a significant effect on the flexural strength of hybrid composite.