This study investigated GPS TEC fluctuations over the high-latitude site, Tromsø, Norway (69.66°N, 18.94°E), in the solar minimum 2007 - 2008. The TEC fluctuation index Fp that defined by Mendillo et al. (Radio Science 2000) was adopted to quantify TEC fluctuations, in which 50 ≤ Fp < 200 and Fp ≥ 200 respectively represent moderate and strong irregularities. The investigations include the seasonal and temporal variation of Fp; the correlation between Fp and the magnetic indices Kp and AE; the comparisons between Fp and the ionospheric electron density observed by the Tromsø incoherent scatter radar and COSMIC. The results are that Fp ≥ 50 occurred frequently in all seasons but Fp ≥ 200 occurred more frequently in the equinoctial months; Fp ≥ 50 mainly occurred in 18 - 04 LT, and maximized around 22 LT in the equinoctial months. The linear correlation between Fp and Kp was poor but that between Fp and AE was moderate. The maximal Fp and the percentages of Fp ≥ 50 and Fp ≥ 200 increased with Kp and AE; Fp ≥ 200 is nearly negligible when Kp < 4. The high electron density structure that resulted from the auroral activity caused Fp ≥ 50 when it located in the E region or extended from the E region to the F region. The findings are that Fp ≥ 50 at auroral region mainly relates to the auroral activity. The Fp seasonal variation can be explained by the effect of sunlight and the geometry of the magnetotail. Occurrence of irregularities and their maximal intensity are increased with Kp and AE. Strong irregularities almost only occur in the magnetic disturbance period. Irregularities in the E region or in the E and F regions both can cause Fp ≥ 50.