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Background and Objectives: Wood as a renewable material, with its various benefits, has always had different applications in human society. Humans have always considered wood as a natural and biodegradable material throughout history. However, wood is a natural material originating from different trees creates limitations on its properties such as moisture absorption, dimensional changes, biodegradation, degradation by ultraviolet radiation and weathering, etc. Therefore, this natural product may need to be modified to achieve the desired functionality and application. Different modification methods can improve the physical and mechanical properties of wood. Enhancement of light wood density by compression process is one of the wood modification methods that often improve the mechanical properties. However, spring back is the main subject in this process. There have been done numerous studies in this field, but no suitable method has been presented until now that can effectively control spring back. Therefore, the aim of this study was the improvement of the physical and mechanical properties of poplar wood (Populus deltoides) by using hydrothermal pretreatments; at the same time, spring back assessment under this process. Materials and Methods: In the current study, Poplar wood (Populous deltoids) was initially cut into the timber in the wood and paper laboratory of the Alborz research complex and then changed into the pieces with dimensions of 45×7×4.2 cm3 (longitudinal × tangential × radial). Afterwards, the wood specimens were hydrothermally treated at temperatures of 100 °C for three times of 0, 60, and 120 minutes. In the next step, the second pretreatment and also densification process was carried out under a hot press in the radial direction with a compression set of 40 percent. The pretreated poplar wood specimens were immediately compressed by the hot press at a temperature of 160, 180, and 200 °C for two times of 60 and 90 minutes. The press pressure was considered 50 kg/cm2 . The specimens were then physically and mechanically tested as density gradient, spring back after press and also conditioning, impact strength and compression parallel to the grain. The results were analyzed based on a complete randomized design (CRD) under a factorial experiment by SPSS software. The mean comparison was evaluated using Duncan's multiple range tests. Results: Results assessment compared with other studies showed that this process, such as uniform structure and less density gradient, could reach acceptable physical and mechanical properties. Moreover, the results revealed a reduction of spring back after press and conditioning and improved mechanical properties such as impact strength and compression parallel to the grain. Conclusion: The densification process of low-density wood species with hydrothermal pretreatments improves the physical and mechanical properties and expands their functional domain. [ABSTRACT FROM AUTHOR]