A fan blade structure includes a hub, an annular partition surrounding the hub, a first blade group and a second blade group. The hub has a top surface and a flank connected to the top surface. The first blade group, disposed on one side of the annular partition, includes two blade arrays having multiple first and second blades respectively. The clearance between the two adjacent first blades is less than that between the two adjacent second blades. The second blade group, disposed on another side of the annular partition, includes another two blade arrays having a plurality of third and fourth blades respectively. The clearance between the two adjacent third blades is less than that between the two fourth blades adjacent to each other.