Disclosed herein is a method of measuring a dielectric constant of a Printed Circuit Board (PCB) for a Rambus Inline Memory Module (RIMM), which includes the steps of measuring a length of a Rambus product of a PCB, applying an input waveform to the Rambus product at a certain probing position and obtaining a cross point of rising times of the input waveform and an output waveform generated by reflection of the input waveform, obtaining time corresponding to the cross point, and calculating a dielectric constant by substituting the measured length of the Rambus product and the obtained time for corresponding variables of a dielectric constant calculating equation.