Biaxially oriented polyester film

A biaxially oriented film of a film-forming aromatic polyester derived from an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component, said film having surface properties such that

• (1) it has a surface flatness, R.sub.CLA defined by the following equation, of from 0.001 to 0.016 micron, ##EQU7## wherein R.sub.CLA is the flatness of the film surface determined by the center line average method, f(x) is the sum of vertical distances from a straight line with a length L drawn across a film surface roughness curve measured by using a needle-contacting type surface roughness tester and being nearly parallel to the base line of said curve to maximum and minimum points of said curve respectively, and L is the length stated above for use in calculating f(x) in the surface roughness curve,
• (2) it has a coefficient of travelling friction, .mu.k, defined by the following equation, of from 0.01 to 0.20, ##EQU8## wherein T.sub.2 is the tension of the film which has been caused to travel on a fixed rod of SUS 304 stainless steel having an outside diameter of 5 mm, and T.sub.1 is its tension before passage over the fixed rod, and
• (3) the following relation is established between R.sub.CLA and .mu.k,
• 0. 10.ltoreq.10.times.R.sub.CLA +.mu.k.ltoreq.0.31.