In this paper,the SITEC phenomena is investigated which were caused by the X-class X-ray solar flares occurred from 1996 to 2003.The TEC data which obtained from a global GPS network are used to calculate the TEC increment,ΔTEC
f,and the variation rate increment,Δ(dTEC
f/d
t),caused by solar flares.It is found that both ΔTEC
f and Δ(dTEC
f/d
t) are closely related with the flare's maximal X-ray flux and its location on the solar disc.After eliminating the effect of the maximal X-ray fluxes and the locations on solar disc of different flares,results showed that there is a negative relationship between ΔTEC
f and the distance between the Earth and the Sun.The larger the distance,the smaller is the ΔTEC
f.Another finding is that Δ(dTEC
f/d
t) correlates with the duration of the flare.Some characteristics of the flares which cause no obvious SITEC are also found.