Thorn-like polycrystalline Ca₂B₂O₅.H₂O microspheres with nano-sized slices were synthesized using boric acid and calcium hydroxide as reactants by a facile catalyst-free hydrothermal method at low temperature. The products were characterized by means of X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The XRD pattern reveals that the Ca₂B₂O₅.H₂O is a monoclinic phase polycrystalline with cell parameters 𝑎 = 0.6702, 𝑏 = 0.5419 and 𝑐 = 0.3558 nm. SEM also reveals that the monoclinic phase polycrystalline are thornlike microspheres composed of many flakes with an average thickness of <100 nm. Possible reaction and growth mechanism were also discussed.