There is provided a polyester resin composition excellent in mechanical properties, heat deformation temperature, surface appearance, dimensional stability and mold releasing ability, and a process for preparing the same. The polyester resin composition contains a thermoplastic polyester resin and layered phyllosilicate, wherein the layered phyllosilicate in the resin composition satisfies at least one of the following conditions (a) to (c):(a) ratio of layered phyllosilicate having equivalent area circle diameter [D] of at most 3,000 Å is at least 20%;(b) a mean value of equivalent area circle diameter [D] is at most 5000 Å;(c) the number of particles [N] per unit ratio of layered phyllosilicate present in 100 μm2 of a resin composition is at least 30.