A heterodimeric Fc-fused protein and a pharmaceutical composition comprising the heterodimeric Fc-fused protein are disclosed. The heterodimeric Fc-fused protein includes first and second Fc regions of an immunoglobulin heavy chain constant region (Fc) pair and in which IL-21 is bound to at least one of the N-terminus or the C-terminus of the first Fc region and/or the second Fc region, wherein CH3 domains of the first Fc region and the second Fc region are mutated such that the formation of a heterodimer is promoted. When the heterodimeric Fc-fused protein is used, an in vivo half-life of IL-21 included in the heterodimeric Fc-fused protein may be significantly increased.