In the research carried out, the mechanical properties of M303 steel are evaluated in its supplied state and after heat treatments of quenching between 800 °C and 900 °C followed by tempering between 350 °C and 500 °C. For the manufacturing and experimentation process, ASTM E8 and ASTM E18 standards were used, complying with the indicated experimental indications. To select the specimens and determine their acceptable quality level, AQL tables were generated. After the thermal treatments of quenching and quenching-tempering, the increase in its mechanical properties was verified, such as: hardness by 39.2%, yield stress by 6.68% with respect to the quenching process, however, properties decreased such as elongation by 18.53%, causing a lower breaking point compared to its supply state. This research contributes to the existing body of knowledge by providing specific temperature range optimization for M303 steel heat treatments, filling a gap in literature where precise tempering parameters for this particular alloy are limited. From a practical standpoint, the established relationship between the quenching-tempering process and the mechanical properties offers manufacturers direct guidance for tailoring M303 steel performance to specific industrial applications, particularly in plastic mold manufacturing where surface hardness and dimensional stability are critical requirements.