Abstract: The material properties of hot rolled plain carbon steel were studied using boundary effect theory, linked to the properties of toughness and strength. The method for determining the fracture toughness and yield strength of hot rolled plain carbon steel was proposed. The formulas for design parameters ae and σ_n were obtained. Based on the experimental research on the identical size steel plates of Q235 under tension with different a-ratio=a₀/W (ratio of initial cracks a₀ and plate size W), the reasonability and availability of this model and appropriate method were experimentally confirmed. The plain stress fracture toughness and yield strength can be measured through the small size single edge notch test (SENT) hot rolled plain carbon steel specimens of a single size with different initial cracks. On top of that, the specimens using this proposed method do not meet the size, pattern and loading requirement from current codes, and do not need the fatigue cracks.