First principle method was used to investigate the complex dielectric function of CuInSe2, which agrees well with the experimental results.Optical properties of CuInSe2 nano-film have been calculated and compared with those of bulk CuInSe2 basing on complex dielectric function calculation.It is found that bulk CuInSe2 has the biggest static dielectric constant (about 13), while nano-films have the lower values (about 6 to 9).The optical properties of the thinnest film (11.6 (A) thickness) are apparently different from those of the bulk.As the thickness increases (23.2 (A) and 34.8 (A) thickness), the nano-films have similar but still different optical properties from those of the bulk.We also found that the band gap decreases with increasing thickness of the nano-film.