The aerosol ionic composition of P $M_{2.5}$ measured at Gosan, Jeju Island, Korea, for 4 years between March 1998 and February 2002 are presented and discussed. The annual mean concentration of non- sea-salt sulfate (nss -S $O_4$$^{2-}$) and ammonium (N $H_4$$^{+}$) ions are high (0.094 $mu$eq/㎥, and 0.085 $mu$eq/㎥, respectively). Also, nss-S $O_4$$^{2-}$ and N $H_4$$^{+}$ show high correlation (0.892). The concentrations of most ions are high in springtime. As the result of factor analysis, Gosan area mainly affected by sea-salt, anthropogenic species, and crustal species.ies.