The present invention relates to a balance structure of a fan rotor, which comprises a fan wheel and a balancing part. The fan wheel has a hub and a plurality of blades disposed on the circumferential side of the hub. The hub has a top wall connected to a sidewall. An unbalanced portion is selectively disposed on the top wall or on the sidewall. The unbalanced portion is disposed on one side of a symmetrical axis of the hub. The balancing part is disposed at the unbalanced portion such that the fan rotor is balanced.