In this paper, a three dimensional ionospheric electron (Ne) density model derived from FormoSat3/COSMIC GPS Radio Occultation measurements, called the TaiWan Ionospheric Model (TWIM), was used to determine ionospheric delay for GPS single-frequency positioning. The Ne profiles were used to calculate for the slant TEC (STEC) between a receiver and each GPS satellite. These derived STEC were used to determine the ionospheric delay on the L1 frequency as measured by single-frequency receiver and is used to correct the pseudorange single-frequency observations. The corrected pseudorange for every epoch was used to calculate the position of the receiver. Initial calculations showed that the TWIM can improve positioning to about 90% during daytime and to about 50% during nighttime as compared to uncorrected positions. It was also shown that, in general, it provides better positioning than other ionospheric models, such as the Global Ionosphere Map (GIM), and Klobuchar model, especially during daytime.