The tensile strength of nuclear graphite is an important parameter in evaluating and calculating the structural strength of High Temperature Reactor-Pebbled Modules (HTR-PM). The tensile strength of nuclear graphite with varied sizes and surface roughness was tested, and the size effect tensile strength formula was created. The results reveal that when the size of nuclear graphite increases, its tensile strength falls. However, in the 4 mm to 8 mm range, the size impact is less noticeable. The Weibull modulus m0.95 value is calculated based on the experimental data, and the size effect formula is established. The difference between the estimated value and the experimental value computed using the size effect formula is less than 10%. Tensile strength may be calculated using the size effect formula for nuclear graphite specimens of various sizes. The impact of surface roughness on tensile strength is linked to the grinding direction and specimen size. After a certain level of surface roughness, the tensile strength diminishes when the surface scratches are perpendicular to the tensile force. The stronger the effect, the smaller the specimen size. When surface scratches are parallel to the tensile tension, the tensile strength does not vary much as the surface roughness increases.