Abstract The plastic deformation behavior of single crystals of two transition-metal diborides, ZrB 2 and TiB 2 with the AlB 2 structure has been investigated at room temperature as a function of crystal orientation and specimen size by micropillar compression tests. Although plastic flow is not observed at all for their bulk single crystals at room temperature, plastic flow is successfully observed at room temperature by the operation of slip on {1 $${\bar{1}}$$ 1 ¯ 00}<11 $${\bar{2}}$$ 2 ¯ 3> in ZrB 2 and by the operation of slip on {1 $${\bar{1}}$$ 1 ¯ 00} and {1 $${\bar{1}}$$ 1 ¯ 00}<11 $${\bar{2}}$$ 2 ¯ 0> in TiB 2 . Critical resolve shear stress values at room temperature are very high, exceeding 1 GPa for all observed slip systems; 3.01 GPa for {1 $${\bar{1}}$$ 1 ¯ 00}<11 $${\bar{2}}$$ 2 ¯ 3> slip in ZrB 2 and 1.72 GPa and 5.17 GPa, respectively for {1 $${\bar{1}}$$ 1 ¯ 00} and {1 $${\bar{1}}$$ 1 ¯ 00}<11 $${\bar{2}}$$ 2 ¯ 0> slip in TiB 2 . The identified operative slip systems and their CRSS values are discussed in comparison with those identified in the corresponding bulk single crystals at high temperatures and those inferred from micro-hardness anisotropy in the early studies.