GPS has been an important tool to determine an accurate position on the Earth's surface. However, the accuracy is marked by error sources, i.e. the ionospheric effects. A 300 m error can be introduced a geomagnetic storm. This error can be corrected by processing the GPS data if it is known. Ionospheric delay has repeated trends and seasonal patterns, so we adopt the Holt-Winter method to forecast ionospheric delay due to its ability to forecast time series with repeated trends and seasonal patterns that suit the ionospheric delay variations. The GPS observation data in 2005 from Parit Raja Station (10 52' N, 1030 06' E), Malaysia were used. Results show that the error between the forecast and the real ionospheric delay value is about 6%. Therefore, the error correction for ionospheric delay can be made for more than 90% of the total error.