Abstract: The present invention relates to a process for preparing a polyethylene naphthalate possessed with high viscosity, characterized by sloving of various problems of the prior art by controlling the rate of the polycondensation reaction.Particularly, the present invention provides a process for preparing polyethylene naphthalate from an esterification reaction product of naphtalene dicarboxylic acid or an alkyl ester thereof and a diol, which comprises polycondensing the esterification reaction product in the presence of a polycondensation catalyst in two steps wherein the first step is carried out at a pressure ranging from 500 to 30 torr and the second step is carried out at a pressure ranging from 10 to 0.1 torr with controlling the reaction rate such that the differential increase of the intrinsic viscosity of the resultant polymer can satisfy the following equations:IV(t.sub.1)-IV(t.sub.0)<0.4 dl/g (1)IV(t.sub.2)-IV(t.sub.0)>0.3 dl/g (2)wherein: IV(t.sub.

Abstract: The present invention relates to a biaxially oriented polyester film comprising a metallic alkylbenzene sulfonate derivative of formula(I), polyethylene glycol and, optionally, bis(4-.beta.-hydroxyalkoxyphenyl) sulfones of formula(II), which has improved antistatic property, clarity, mechanical properties and heat resistance: ##STR1## wherein: R represents an optionally substituted C.sub.8-25 alkyl group;M represents an alkali or alkali earth metal;m is an integer of 100 or fewer; andn is an integer from 1 to 10.