Abstract: This paper describes the details of the procedure of deriving instrumental stellar magnitudes from a CCD image using aperture photometry. We conducted experiments of the procedure based on CCD images obtained with the 1 m telescope of the Yunnan Observatory. The results reveal that the internal precisions are 0.003mag and 0.2mag for a bright star (~10mag) and a faint star (~17mag), respectively. In addition, we discuss related issues and some experiences from the experiments. The comparison to the results from the photometric software IRAF is presented as well.