Complexity of fault systems in Japan and the Philippines is quantitatively measured using fractal dimension. The areas studied are the Median Tectonic Line, Izu, Unzen on the Japanese Islands, and the fault system yielded by the Philippine earthquake which occurred on July 16, 1990. Results are summarized as follows: Fractal dimensions along the Median Tectonic Line vary between 1 and 1.3. The Unzen area gives the highest fractal dimension (1.4) of all the areas studied, which implies that the fault system there is the most complex. Fractal dimension of the fault system associated with the Philippine earthquake is smaller than 1. It can be interpreted that this fault system is almost one-dimensional, with truncated numerous gaps of various sizes. ¿ American Geophysical Union 1992