The mass resolution of Medium Energy Ion Scattering Spectroscopy (MEIS) was improved by using $Li^+$ ion instead of $H^+$ ion. Scattering cross sections for the screened potential between $Li^+$ ions and target atoms were obtained by multiplying the Rutherford's formula by the correction factor $f$. The correction factor was obtained by integrating Ziegler-Biersack-Littmark(ZBL) potential curve between $Li^+$ ion and target atom. With $Li^+$ ion MEIS, the composition of the surface of Pt-Co alloy thin films could be analyzed quantitatively. A 1000Å $Pt_50Co_50$ thin film was sputtered with 3keV $Ar^+$ ions and analyzed using medium energy $Li^+$ ions. It was observed that Co was preferentially sputtered from the surface. The ratio of Pt and Co at the several surface layers was 80 : 20. The sample was annealed at 600℃. The annealed sample showed extensive silicide formations. The native oxide layer of GaAs(001) was also analyzed using 120keV $Li^+$ ion and $^69Ga$, $^71Ga$ and $^75As$ could be distinguished. The ratio of Ga and As in native oxide was about 50 : 50. The GaAs(001) covered with native oxide layer was annealed at 600$^{\circ}$C and then, the ratio of Ga and As was about 55 : 45. The GaAs was also sputtered with 3keV $Ar^+$ and $N_2^+$ and the altered layers were formed. The layer of the former consisted of Ga 50% and As 50%. The layer of the latter consisted of Ga 70% and As 30%.
