This paper presents two circuits to measure pulse width distribution of single event transients (SETs). We first review requirements for SET measurment in accelerated neutron radiation test, and point out problems in previous works, such as time resolution, time/area inefficiency for obtaining large samples, and uncertainty in absolute values of pulse width. We then devise two measurement circuits and a pulse generator circuit that satisfy all the requirements and attain sub-FO1-inverter-delay resolution, and propose a measurement procedure for assuring the absolute width values. The operation of one of the proposed circuits was confirmed by a preliminary radiation experiment of alpha particles with a fabricated test chip.