Angular and temperature dependence of the ESR spectrum of a single crystal of a salt, MEM–[TCNQ] 2 , were measured under uniaxial pressure up to 1kbar by using a hand-made pressure cell and a conventional ESR instrument in the temperature range of 1.5–300K. The ESR spectrum is deformed with increasing pressure and the deformed ESR spectrum significantly depends on the temperature and orientation of the crystal. The ESR spectrum at low temperatures is well separated into three absorptions. One of the absorptions seems to result from free spins and the other two absorptions seem to result from spin-pairs.