A high pressure and low temperature x-ray experiment was performed to study the isotope effect on the phase transition of KH3(SeO3)2 (KTS). By using a single crystal of KTS in a diamond anvil pressure cell, temperature and pressure dependences of the order parameters (spontaneous strain Δα) in the lower symmetry phase and the lattice parameters in the higher symmetry phase are precisely measured. The phase transition temperature Tc reduces to zero when pressure is raised up to 1.3 Gpa, while the distance between two oxigens at P=1.3 Gpa and T=10K is estimated to be 2.47 Å. These experimental results are explained by taking the geometrical isotope effect on the interaction energy J into account. The tunneling frequency \\varOmega does not seem to play any important role on Tc.