Abstract: Method and apparatus for estimating the performance of a gas tube. The method comprising the steps of: (a) preparing a sample tube to be analyzed and cutting the sample tube in a desired size and shape; (b) examining distribution of defects and surface condition of the cut sample tube with an optical microscope; (c) analyzing structure and composition of surface defects which can not be measured in the step (b), to determine type and composition of the surface defects and shape of a surface grain; (d) analyzing structure of an inner surface-treated layer of the sample tube along the thickness thereof; and (e) synthetically analyzing data for defect density and surface roughness, which are numerically expressed through the steps (a) to (d), to define a reference data which can be used in a semiconductor manufacturing process.