High pressure properties of isostructural anion radical salts Me 4 X(CPDT-TCNQ) 2 (X=N, P and As) having extremely one-dimensional electronic structures have been investigated by resistivity and X-ray diffraction measurements. The metal-insulator transition temperature shifted to higher temperature with increasing pressure up to around 9kbar. The abrupt increases in the resistivity at 9.4 and 7.0kbar correspond to the appearance of superlattice reflections at room temperature for the Me 4 P and Me 4 As salts, respectively. The superlattice reflections were also observed above 7.9kbar on the Me 4 N salt. The origins of these transitions are thought to be the 2k F CDW for the Me 4 N and Me 4 P salts, and the 4k F CDW for the Me 4 As salt. In addition, the 4k F to 2k F CDW transition is observed at 10.4kbar for the Me 4 As salt.