The objective of this work was to evaluate the physical and mechanical properties of wood–plastic composites (WPCs) treated with anti-microbial agents. Specimens having 60 wt.% wood flour (WF) were mixed with high density polyethylene (HDPE, 38 wt.%) and three different additives, namely maleic anhydride grafted polyethylene (MAPE, 2 wt.%) as coupling agent, 3-iodo-2-propynyl butylcarbamate (IPBC, 0.3, 0.6 and 0.9 wt.%) and 2-thiazol-4-yl-1H-benzoimidazole (Irgaguard, 0.3, 0.6 and 0.9 wt.%) as fungicide were prepared. Static mechanical properties of the samples (modulus of rupture – MOR and modulus of elasticity – MOE) were evaluated using static bending tests while impact strength (IS) was studied by Izod impact tests. Physical properties, namely water absorption (WA) and thickness swelling (TS) were also determined. In general, test results showed that the addition of anti-microbial agents tended to reduce both the physical and mechanical properties of samples. Based on the initial findings of this work MOR, MOE and IS of the samples considerably decreased as above preservative chemicals were added to the formulations. In addition, WA and TS were significantly increased when the fungicide content was increased from 0.3% to 0.9% by weight. The results also showed that Irgaguard had more negative effects on physico-mechanical properties compared to IPBC. The possible reason proposed for this kind of behavior may be the weak interfacial adhesion between the MAPE and WF in the presence of the fungicide agents. Another reason is the DMSO solvent which can readily degrade the crystalline structure and swell amorphous regions of cellulose and cause more voids in the WF. Extra voids in the composite also contribute to water absorption.