A delay-compensation circuit for low-power subthreshold digital circuits is proposed. Delay in digital circuits operating in the subthreshold region of MOSFETs changes exponentially with process and temperature variations. Threshold-voltage monitoring and supply-voltage scaling techniques are adopted to mitigate such variations. The variation in the delay can be significantly reduced by monitoring the threshold voltage of a MOSFET in each LSI chip and exploiting the voltage as the supply voltage for subthreshold digital circuits. The supply voltage generated by the threshold voltage monitoring circuit can be regarded as the minimum supply voltage to meet the delay constraint. Monte Carlo SPICE simulations demonstrated that a delay-time variation can be improved from having a log-normal to having a normal distribution. A prototype in a 0.35-µm standard CMOS process showed that the exponential delay variation with temperature of the ring-oscillator frequency in the range from 0.321 to 212kHz can remain by using compensation in the range from 5.26 to 19.2kHz.