The present study examined the effect of prior austenite grain size on twin density and hardness of austenitic stainless steels (ASS). The 253 MA and 316L ASS were subjected to multi-pass cold rolling to reduce thickness up to 2.3 mm. Subsequently, the rolled steels were heat treated at 1100°C at 0, 900, 1800, 2700, and 3600 seconds in a tubular furnace in a hydrogen atmosphere. At the end of the annealing time, the rolled steel was quenched in the cooled zone of the tubular furnace until it reached room temperature in a hydrogen atmosphere. Then, microstructure observation of ASS was done to identify the austenitic grain size and annealing twin, and a hardness test was performed using the micro-Vickers hardness scale. The line intercept method was used to measure the changes in 253 MA and 316L austenitic grain sizes. ImageJ software was used to measure grain size and twin length. The results showed that austenite grains of both steels grew normally; 253 MA ASS had a lower SFE and K value than 316L ASS, which indicated that 253 MA ASS had sluggish grain growth, smaller grains, more easily formed annealing twins, and higher twin density. The Hall–Petch coefficient, K’, of 253 MA ASS was higher than 316L ASS, which resulted in a higher hardness value. The Sellars, Pande and Hall-Petch models were shown to predict austenite grain sizes, twin density, and hardness in 253 MA and 316L ASS.