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Gate-all-around (GAA) MOSFETs with elliptical cross section are studied in this letter. From the solution of the Poisson equation, an effective radius concept (Reff) is proposed to convert elliptical GAA MOSFETs into equivalent circular GAA MOSFETs for the study of short-channel effect. The Reff approach is used to estimate the subthreshold swing, drain-induced barrier lowering, and Qon/Qoff ratio of elliptical GAA MOSFETs. The predictions are validated by numerical 3-D TCAD simulations, and good agreement is achieved. Reff is also simplified to an area-to-perimeter ratio of the channel under some special conditions, and its physical meaning is explained.