To investigate the size effect on the fire behavior of reinforced concrete frame structures， fire tests on six single-story reinforced concrete plane frame structures with similar geometric characteristics but different scales were carried out under standard fire conditions. Then， finite element analysis and parametric studies were conducted to evaluate the effect of the column’s axial compression ratio and beam’s load ratio on the failure modes， deformation development， and fire resistance of the frame structures with different scales. The results show that there is a significant size effect on the fire resistance of geometrically similar frames under the same fire conditions and similar loading levels. The fire resistance increases as the sizes of the specimens increase， but the increasing extent is less than that predicted by the similarity （or equivalent） relationship of the fire resistance of the components proposed by previous studies. The failure mode of reinforced concrete plane frame structure under fire is greatly affected by the beam’s load ratio and the column’s axial compression ratio. When the column’s axial compression ratio is lower but the beam’s load ratio is higher， the frame structure is prone to beam failure， and otherwise it is prone to column failure. Due to the size effect on the columns being more significant than the size effect on the beams under fire， when column failure occurs in small-sized frames， beam failure may occur in large-sized frames with similar geometry and load levels.