Dual-frequency GPS receiver can automatically correct time delay error caused by ionospheric Total Electron Content (TEC). But it cannot eliminate signal scintillation error caused by ionospheric irregularities. Even for differential GPS systems, ionospheric error is one of major sources in GPS positioning, for example, error from ionospheric TEC gradient, which will affect accuracy and degrade performance of GPS application system. In this paper, the magnitude of ionospheric TEC horizontal gradient is studied by means of GPS observations. The vertical TEC, and its horizontal gradient and ROTI index over NTUS station, which located in equatorial anomaly region, are derived from GPS observation provided by IGS network. It is found that, between post-sunset and around midnight, the ionospheric TEC has great fluctuation, and intense radio waves scintillation, which is represented by big ROTI index, takes place during this periods. Furthermore, the steep TEC horizontal gradient and its fast variation with respect to time appear. By comparison, the ROTI index, ionospheric TEC horizontal gradient and vertical TEC correlate very well with each other, and their variations characteristics result from density irregularities produced by plasma instability in ionosphere. It is suggested that when scintillation events are investigated, ionospheric TEC gradient is also one of considerable parameters.