Abstract: The present invention describes a tester for determining the value of unknown delay lines in the nanosecond and subnanosecond range. The unknown delay line is introduced into the circuit loop of a free-running square wave oscillator previously calibrated to a predetermined frequency. A change in oscillator frequency occurs which is a measure of the value of the delay introduced into the loop by the unknown line. Such delay may be calculated as a function of the calibration and delay line-measured frequencies. The present tester provides measurements with increased accuracy and rapidity over previous measuring techniques.