A huge softening of the transverse elastic modulus C66 has been found in UCu2Sn around a phase transition point at ∼16 K. In addition to the elastic moduli, the specific heat and x-ray diffraction were measured for polycrystals and single crystals in order to elucidate the origin for the transition. All anomalies obtained in the thermodynamic properties were analyzed by assuming a localized 5f2 configuration in the hexagonal symmetry. It is revealed that the transition originates from a quadrupolar ordering of the non-Kramers ground doublet Γ5.