A connection structure is applied to a serial fan assembly, which includes an upstream fan and a downstream fan. The upstream fan and the downstream fan have different sizes. The connection structure connects the upstream fan and the downstream fan so that the upstream fan and the downstream fan are arranged in series. The connection structure has a housing, a base and a plurality of ribs for connecting the base, and the housing is formed with an air guiding passage and a chamber. The upstream fan or the downstream fan is accommodated in the chamber or the air guiding passage is located between the upstream fan and the downstream fan.