To analyze the generalized Brownian motion, i.e. the fractional Browni
an motion, we propose a path integral which is governed by the modifie
d action along the principal path with fractal natures, i.e. mainly ob
servable path in a diffusive phenomena. By modified the definition of
the action and summing over fractal paths, the path integral is derive
d. We investigate several properties of this integral. The principal p
ath has a fractal structure and the path integral represents the trans
ition probability of the fractional Brownian motion. The transition pr
obability itself has no dependence on the structure of principal paths
. The path integral is mainly characterized by two parameters, the Hau
sdorff dimension D-H of principal paths representing a microscopic str
ucture and the Hurst coefficient H representing a macroscopic structur
e, which are independent of each other.