A method of detecting defects of a photoresist pattern includes generating a scanning electron microscope (SEM) image of a surface of a photoresist pattern and signal intensity data relative to pixel position of the surface of the photoresist pattern. The method also includes setting a lower reference intensity threshold value and an upper reference intensity threshold value used as reference values for detecting defects. The method further includes classifying a pixel position of the signal intensity data having a signal intensity value which is less than the lower reference intensity threshold value or greater than the upper reference intensity threshold value as a defect position.