Background and objectives: Wood treatment with chemicals, especially maleic anhydride, with reducing the number of hydroxyl groups and swelling the cell wall, can be the result of reducing the moisture absorption and dimensional changes. Little evidence is presented about the effect of different conditions on wood treatment reaction with anhydrides. It is likely that the increase in a reaction temperature or time of treatment, with improvement in the hydroxyl groups substitution of the wood cell wall, has a favorable effect on the physical properties of wood. This research was conducted to determine the optimum conditions of temperature and time of treatment with Maleic anhydride and physical properties of treated poplar wood.Materials and methods: Test specimens were prepared according to ASTMD4446-05 standard. Preparations maleic anhydride solution was done with a concentration of 20% w / v in acetone as a solvent. Specimens were impregnated with maleic anhydride using vacuum-pressure method with experimental cylinder. For determination of optimum level of temperature and time, treated specimens were heated in an oven for two period time of 4 and 8h and two temperature levels of 120 and 150ºC Then, treated specimens were subjected to soaking-drying test. Finally, samples weighing and weight gain due to reaction with maleic anhydride and hydroxyl groups substitution was determined. In order to determine the optimal level of treatment, the samples were tested in four consecutive soaking-drying cycles. In each cycle, weight change and hydroxyl groups substitution were measured. At each stage of soaking- drying cycle, physical properties were easured. Also, to evaluate the chemical structure and morphology of control and treated samples, the spectroscopy test and electron microscopy images were used.Results: SEM images with cell wall swelling in treatment with maleic anhydride confirmed the reaction. Changes in weight gain, substitution of hydroxyl groups, water absorption and dimensional stability subjected to soaking-drying test were determined. Based on the results, the effect of reaction time on the measured properties was not significant. After Soaking-drying cycles, the higher reaction temperature with formation a more stable structure against hydrolysis, resulted to retain the weight gain of the treatment. In the fourth period of immersion, the highest substitution of hydroxyl groups, hydrophobicity and anti-swelling efficiency of treated specimens was observed at 150°C. Water absorption of specimens treated with maleic anhydride at 150°C decreased 61.76% compared with the control.Conclusion: Based on morphology, in treated wood, despite the increase in weight and improved anti-swelling effect, due to the entrance of maleic anhydride molecules into the cell wall, porous structure was maintained. Treatment improves hydrophobicity and dimensional stability of the treated samples. Increase reaction time treatment from 4 to 8 hours, did not cause significant differences in the measured properties, and this was confirmed in the FTIR spectrum, but a rise in temperature by effect on the connection method to polymer constituent of the wood cell wall, resulted in significant improvements in treatment reaction. In a general, conclusion can be stated that the increase in temperature of the treatment reaction with maleic anhydride, with forming more stable structure against hydrolysis, will lead to efficient stabilization.