The present invention relates to an insulin conjugate having improved in vivo duration and stability, which is prepared by covalently linking insulin with an immunoglobulin Fc region via a non-peptidyl polymer, a long-acting formulation comprising the same, and a preparation method thereof. The insulin conjugate of the present invention maintains in vivo activity of the peptide at a relatively high level and remarkably increases the serum half-life thereof, thereby greatly improving drug compliance upon insulin treatment.