This study was carried out to investigate the physical properties and decay resistance of beech wood treated with natural pistachio resin (PR) from Iranian wild pistachio trees (Pistacia atlantica), alone and in combination with boric acid (BA). Wood samples were impregnated with different concentration of PR dissolved in ethanol (3 to 20 %) with vacuum-pressure technology. The combination of PR (20 %) and BA (2 %) was also conducted to evaluate any interaction or synergistic effects. The water absorption, volumetric swelling, and decay resistance against Trametes versicolor fungi, before and after a leaching test (EN 84), were measured on treated and untreated samples. The chemical compositions of PR were also identified by gas chromatography–mass spectrometry (GC-MS) techniques. The chemicals analysis identified more than 20 different compounds in the PR, monoterpenoids being the predominant fraction and α-pinene the major component. The samples treated with a higher concentration of PR showed much higher weight gain percentage (WG%). The results showed that the increase in WG% reduced the average values of water absorption and volumetric swelling of treated samples even after long terms of soaking in water. The decay resistance of the treated samples increased against white rotting fungi as the values of WG% increased. Efficient protection was seen when a combined treatment of PR and BA was used. Even after the leaching process, the weight loss of the treated samples was less than 3 percent. The samples treated with BA alone largely lost their effectiveness against fungal attack after the leaching. The use of PR along with an environmental friendly co-biocide can also be recommended for wood preservation in places that require minimal toxicity.