The Fermat point of a triangle is the point with the minimal total distance from the three vertices of the triangle. Meanwhile, the total distance from the three vertices to the Fermat point is called the Fermat distance. In this paper, we discuss the Fermat distance on some networks. We study the mean Fermat distance of an unweighted and undirected hierarchical network GnGn, which is obtained by analytical method and iterative calculation. We then reveal the relation between the mean Fermat distance ˉℱ and the mean geodesic distance ˉd in general networks, namely, 32ˉd≤ˉℱ≤2ˉd. The ratio of the mean Fermat distance to the mean geodesic distance of Gn tends to 3/2, which is the lower bound of the inequality above. Moreover, the result shows the small-world effect of Gn. Finally, we illustrate that the mean Fermat distance is significant in both small-world and scale-free networks.