The precipitable water vapor (PWV) can be estimated from dual-frequency GPS observations, which has been widely used for atmospheric study and weather forecasting. However. high-order ionospheric delays are normally ignored in GPS applications. In this paper, high-order ionospheric effects on GPS-estimated precipitable water vapor are investigated from 30 days of GPS data in June 2011 at 8 GPS stations in Turkey. Firstly, the second- and third-order effects on GPS data are corrected in RINEX using IGRF11 model. and then the precipitable water vapor (PWV) is obtained from raw and corrected RINEX data with GAMIT software. Results show that high-order ionospheric effects arc up to 1 millimeter on PWV during this period. Solar activity is an important effect on ionospheric activities. When data during the peak solar activity days arc used for same stations in 2011, high-order ionospheric effects on PWV is increasing, while low solar activities, high-order ionospheric effects arc much smaller.