Abstract: Background and Objective: Grass pea (Lathyrus sativus L.) is a multipurpose grain legume with an indeterminate growth habit. The objective of this study was to identify adaptation mechanisms of grass pea to water deficits. Materials and Methods: In the germination experiment, the used osmotic potential were: 0 (distilled water); -0.03, -0.1, -0.7 and -1.0 MPa, osmotic shock induced by increasing concentrations of polyethylene glycol (PEG). After treatment for 7 days, germination percentage, root and shoot length, seedling fresh and dry weight were measured. In the second experiment, water deficit was imposed on plants of grass pea by withholding water from first flowering. Results: Seeds of L. sativus were able to germinate at all concentrations of PEG treatments. Higher concentrations of PEG (-1.0 MPa) were less inhibitor effect on seedling shoot and root elongation. This study showed that under water deficit, grass pea avoids dehydration through a reduction in leaf area and fresh biomass. Water deficit reduced dry matter, seed yield, seed number, seed size, flowers number and harvest index, respectively, when compared with the controls. Conclusion: Conclusively, grass pea showed a better germination under osmotic stress; therefore, it seems to be a suitable candidate for breeding programs.