The effects of a silane coupling agent such as (N‐(n‐butyl)−3‐aminopropyltrimethoxy silane) on old corrugated container (OCC) fibers, as a recyclable material, were studied on the ultimate strength of the prepared OCC fibers/high density polyethylene composites. Raw materials were mixed in a counter‐rotating twin‐screw extruder and injection molding machine was used to make samples. Various characterization methods, based on ASTM standard, were conducted to evaluate physical properties (water absorption and thickness swelling) and mechanical behavior (tensile, flexural, impact, hardness, and elongation‐at‐break) of the composites. Also, the thermal properties of the specimens were recorded by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). Finally the results of the treated samples were compared with the non‐treated and pure polymer specimens. The obtained results showed that increasing coupling agents by a maximum of 2 wt% enhanced mechanical behavior (including flexural and tensile strengths), thermal stability, and increased crystallinity of the composites. Moreover, water absorption and thickness swelling of composite materials declined. Morphological studies by scanning electron microscopy methods (SEM) indicated that fiber pull‐out decreased in treated samples and coupling agent facilitated strong adhesion in fiber/polymer interface. The highest flexural strength and modulus were obtained in composites with 2 wt% silane coupling agent and 30 wt% OCC fibers. POLYM. COMPOS., 39:2054–2064, 2018. © 2016 Society of Plastics Engineers