The spinning of poly(ethylene terephthalate) (PET) filaments in laboratory‐scale was studied. The objective was to study the effect of modified calcium silicate (CS) with vinyltriethoxysilane (VTES) on the melt spinning of PET fibers. The CS was modified with VTES (2% v/v) in diethyl ether at room temperature for 24 h. The modification of CS with VTES improved agglomeration of CS, hydrophobic, and heat‐resistance properties. These properties were expected that modified CS could be used as the filler in melt spinning of PET. The incorporation of modified CS in PET was spinnable. The addition of CS in PET improved its heat resistance. Also, the filler had an effect on the mechanical properties of polyester fibers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012