Deformation and fracture behaviors as well as their mechanisms of polycrystalline beryllium at room temperature were systematically studied by in situ tensile test in SEM, characterizing fracture cleavage planes by electron backscattered diffraction (EBSD) technique, and twinning deformation analyzing by OM. The results show that slip and twinning deformation of polycrystalline beryllium are difficult to occur under tensile stress at room temperature. Slip bands happen only in some grains with a favorable orientation, and finally twinning deformation grain number accounts for only about 5% of the total grains. There exists the cross slip between (0001) basal plane and {1010} prismatic plane in the deformation process. Microcracks usually initiate at one grain boundary, then propagate by a transgranular way and terminate at the other side of the grain boundary in polycrystalline beryllium. Crack initiation of polycrystalline beryllium is in accordance with Stroh dislocation pile-up crack theory. The growth of microcracks have to depend on different microcracks merging by cleavage steps or tearing way due to a strong blocking effect of grain boundaries on the microcracks propagation. Basal cleavage planes of polycrystalline beryllium are determined to be (0001) and {1010} planes. Both of them are the main paths of cleavage crack initiation and propagation of polycrystalline beryllium. It is not observed that twinning deformation induces nucleation of microcracks.