The creep-rupture strength of precracked specimens made of S10C carbon steel was investigated at 450°C as an extreme case of the strength of specimens with a sharp notch. Two methods were employed for making precracks. One was by using a rotating bending fatigue testing machine with specimens having a sharp notch, as had often been used by many investigators. The other was by using a micro-cutter to make a circumferential slit as narrow as 0.15mm to a prescribed depth at the notch root of the specimen. This specimen was compressed in the axial direction until the flanks of the slit became close together. All the specimens were annealed in vacuum after the precracking. The latter method was mainly adopted in this study because it afforded easier control of geometry and size of precracks than the former method.The S10C carbon steel, which had high ductility, showed“notch-strengthening”for all the precracks investigated in this study. The notch-strengthening was higher for deeper precrack, and decreased with increasing rupture time. For specimens with a precrack-depth to outer-diameter ratio higher than about 0.1, the notch-strengthening was found to converge to a certain value at rupture time extending for hundreds of hours, irrespective of this ratio. The cracks were observed to propagate itself in a mixed manner of both the transcrystalline and the intercrystalline modes. No noticeable difference was observed at longer rupture time between the two methods of precracking, either in strength or in microscopic behavior of crack propagation. This means that the method of precracking by the use of a micro-cutter is effective for investigating the characteristics of crack propagation.