An initial design study of 5mN hydroxylammonium nitrate (HAN) monopropellant microthruster has been performed. Soft lithography utilizing polydimethylsiloxane (PDMS) as structural material is proposed to fabricate micronozzle in a microfluidic device. The method promises the potential in integrating propellant feeding system and micronozzle through microfluidics and MEMS technology. Electrolytic decomposition of HAN with comparatively low ignition temperature is identified as a suitable mechanism in producing thrust in the system. A numerical analysis has been conducted in predicting the performance of microthruster for a range of Reynolds number, Reε65-1200. It is found that the optimum expander half-angle for current micronozzle design is approximately 17.5 degree for Reε1200. The optimum angle is 20 degree for the case of nozzle throat Reε800. Simulation results also indicate that the performance of HAN monopropellant-based microthruster is comparable to that using hydrogen peroxide and hydrazine while it is more environmental friendly.