This paper presents a design of an on-chip single event transient (SET) pulse width measurement system. The proposed system has been designed and implemented in IHP's 250 nm bulk CMOS technology and is intended for evaluation of SET effects in standard digital library cells. It is composed of an inverter-based target circuit, a pulse stretcher and a processing unit for counting the SET pulses and measuring the SET pulse width. The realized system is based on the combination of best practices from various existing designs, and it has a fairly simple architecture capable to provide reliable SET characterization. It supports serial interfacing with the external data acquisition unit which transfers the acquired data to the personal computer. The preliminary evaluation through the circuit-level simulations has demonstrated that the proposed design can detect and measure the SET pulse widths from around 100 ps up to 3.5 ns, with the measurement resolution of approximately 100 ps.