The present paper studies the sudden increase in total electron content (SITEC) on the ionosphere caused by the very intense solar flare on July 14, 2000. According to the well-known Chapman theory of ionization, we derive the relationship between the temporal variation rate, of the total electron content (TEC) and the flare parameters. It is shown that is proportional to the effective flare radiation flux, If, and inversely proportional to the Chapman function, ch(χ), of the zenith angle χ. TEC data observed by the GPS networks located in China, Southeast Asia and Australia during the very intense solar flare on July 14, 2000 are used to statistically investigate the relation between the observed and ch(χ). The analyses show that the two quantities are inversely proportional to each other, as the theory predicted. The present work shows that GPS observation is a powerful tool for studying solar flare effects on the ionosphere, I.e. The sudden ionospheric disturbances (SIDs). Because of its advantages of high precision, large geographical distribution and good temporal resolution, GPS TEC observation may reveal quantitatively the process of ionospheric disturbances caused by solar flares. Therefore, our results are of significance in the space weather research.