In order to study the phase transition in (CH3)(3)NHCl, the crystal structure of the high temperature phase and the temperature dependence of the lattice parameters were determined by the X-ray diffraction procedures. It is found that the crystal exhibits a monoclinic-to-tetragonal phase transition at 308 K and the space group of the high temperature phase is P4/nmm (Z=2). The !east-squares refinement reveals that the structure is approximately represented by the superposition of the four structures similar to the monoclinic one. The electron density calculated for the three methyl groups shows the four maxima at the positions related by the four fold rotation around the c axis. The result indicates the possibility of the occurrence of the rotational-translational coupling in the tetragonal phase.