Polycrystalline CaIrO3 phase, synthesized in a cubic-anvil type high pressure apparatus at 4 GPa and 1473 K, has been investigated using transmission electron microscopy (TEM). Individual crystallites have a platy crystal habit elongated parallel to the crystallographic a-axis, which is the direction of the edge-shared octahedral chain. The dislocations with Burgers vectors b = <100> and $langle$u0w$rangle$ nucleated in the CaIrO3 phase during the synthesis experiments. The potential slip plane could be (010), the most likely having with the (010)-layered structure of IrO6 octahedrons. The crystal morphology and dislocation microstructures of CaIrO3 should provide important constraints for the discussion of the deformation behaviour and polarization anisotropy of the post-perovskite phase in the lowermost mantle.