High‐purity Si3N4 (with 2.5 wt% glassy SiO2) doped with F was prepared by immersion of the starting powder into dilute HF and hot isostatic pressing without sintering additives, using a glass encapsulation method. Oxygen content and cation impurity content were almost the same for the F‐doped and undoped materials. However, X‐ray fluorescence analysis revealed the order of 100 ppm of F in the doped material, and a considerable amount of F was detected from the amorphous SiO2 phase at grain‐boundary triple points by analytical transmission electron microscopy. High‐resolution electron microscopy found that an amorphous intergranular film was omnipresent in both of the materials, with an equilibrium thickness of 10 ± 1 å. Subcritical crack‐growth resistance and creep resistance at 1400°C were degraded significantly by the presence of F. Internal friction of doped materials exhibited a dear grain‐boundary relaxation peak, which suggested that F was present in the intergranular film at the two‐grain junctions; this decreased the grain‐boundary viscosity considerably. The film thickness of the doped material showed no apparent chemical effects and was explained by taking into account competing repulsive forces acting normal to the film.