Abstract: The present invention relates to a biaxially oriented polyester film for magnetic recording medium having a thickness of thinner than 7 &mgr;m and satisfying the formula 1 ≤ P - ( α ⁢   ⁢ t + α ⁢   ⁢ h ) 10 ≤ 10 wherein &agr;t (×10−6/° C.) is the thermal expansion coefficient of the film in transversal direction, &agr;h (×10−6/%RH) is the humidity expansion coefficient in transversal direction and P (ppm/g) is the transversal shrinkage of the film under the load (grams) applied to the film in machine direction.

Abstract: A biaxially oriented polyester film has a first layer of an aromatic polyester containing inert fine particles having an average particle diameter of 1.0 .mu.m or less in an amount of 0.01 to 2% by weight and a second layer of an aromatic polyester. The ratio (t/d) of the thickness t of the first layer to the average particle diameter d of the inert fine particles is 5 to 30. The exposed surface of the first layer has a dynamic friction coefficient (.mu.k.sub.1) against a metal of 0.15 to 0.50, the exposed surface of the second layer has a dynamic friction coefficient (.mu.k.sub.2) against a metal of 0.24 to 0.60, and the ratio .mu.k.sub.2 /.mu.k.sub.1 is 1.2 to 4.0. The film has a thickness of 2 to 8 .mu.m.

Abstract: A biaxially oriented laminate polyester film consisting of a polyester layer A containing lubricant particles I and II and a polyester layer B laminated on one side of the polyester A, wherein each of the lubricant particles I and II contains non-agglomerated particles in a proportion of 80 to 100%; the lubricant particles I are selected from the group consisting of silica, calcium carbonate and crosslinked polymer particles and have an average particle diameter of 0.3 to 1.0 .mu.m and are contained in an amount of 0.005% by weight or more but less than 0.1% by weight; the lubricant particles II are selected from the group consisting of silica and crosslinked polymer particles and have an average particle diameter which satisfies the following expression (1):1<d.sub.I /d.sub.II .ltoreq.10 (1)wherein d.sub.I is an average particle diameter of the lubricant particles I and d.sub.II is an average particle diameter of the lubricant particles II,and are contained in an amount of 0.