PURPOSE: A method for measuring a dielectric constant of a PCB is provided to measure the dielectric constant by using a time domain reflectometry method without an additional test coupon. CONSTITUTION: A length measurement process is performed to measure a length of a Rambus product of a PCB for RIMM(Rambus DRAM Inline Memory Module)(S21-S22). An intersection between a rising period of an input waveform and a rising period of a reflected output waveform is obtained by applying an input pulse to the Rambus product at a predetermined measurement position(S23). A period corresponding to the intersection is obtained(S24). A dielectric constant is obtained by inputting the measured length of the Rambus product and the obtained period to a dielectric constant calculation formula(S25).
