Although nanostructuring strategies can greatly enhance the hardness of diamond, a large amount of grain boundaries were involved, and little research has been reported on the effect of grain boundaries on the indentation hardness and plastic deformation of diamond. Through molecular dynamics simulations, we have systematically investigated the diamond symmetrical 〈001〉 tilt grain boundaries under Vickers indentations. Our results indicate that the smaller the misorientation angle is, the more favorable for the hardness enhancement of diamond. For the symmetrical 〈001〉 tilt grain boundaries models with misorientation angles smaller than 36.87°, the plastic deformations are mainly resulted from the dislocation propagation mode. For the grain boundaries with misorientation angles larger than 36.87°, the plastic deformation is mainly through the atomic disordering mode, accompanied with the dislocation mode. The effects of grain boundaries on dislocation activities and atomic disordering transformations are systematically discussed. The results presented in this work not only answered the question about the effect of symmetrical 〈001〉 tilt grain boundaries on the hardness of diamond, but also offers fundamental insights for understanding the indentation induced plastic deformation mechanisms of diamond grain boundaries, which could provide reference data for the design of novel nano structured superhard materials. Display omitted •The hardness of ∑41(190) grain boundary model is higher than that of (001) plane of diamond.•Increasing of misorientation angle is unfavorable for the hardness improvement of polycrystalline diamond.•Dislocations activities and atomic disordering transformations are two modes of plastic deformations.•Increasing misorientation angle weakens the inhibition of grain boundary on dislocation and atomic disordering activities.