There is provided a biaxially oriented polyester base film for a magnetic tape for use in a cassette for a digital audio tape recorder. The film has (a) a stress when the biaxially oriented polyester base film is extended in the longitudinal direction by 5 % of at least 12 kg/mm², (b) a residual extension in the longitudinal direction of 0.2 % or less and a residual extension in the transverse direction of 0.3 % or less, (c) a heat shrinkage in the longitudinal direction of 1.5 % or less, (d) at least one surface of which has a surface roughness, Ra, of 25 nm or less, and (e) a film thickness of in the range of from 3 to 12 µm. A magnetic tape for use in a cassette for a digital audio tape recorder is also provided.