Thermal behaviors of electrospun polyacronitrile (PAN) fibers incorporated with graphene nanoplatelets and multiwall carbon nanotubes (MWCNTs) were evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. DSC was used to determine the glass transition temperature (Tg), melting temperature (Tm) and heat flow of the polymeric fibers, while TGA was used to determine the stages of thermal breakdown, weight loss in each stage, thermal stability, and threshold temperatures. Glass transition temperature is an especially important property during the processing of polymers, applications, and storage. Pure PAN fiber has a Tg of 104.09C; however, in the presence of 2 and 4 wt.% of graphene in PAN fibers, Tg values were increased to 105.07C and 105.75C, respectively, and then decreased to 102.82C at 8 wt. % of graphene. Similarly, Tg values of PAN fibers were increased to 105.08C and 108.19C in the presence of 2 and 4 wt. % of MWCNTs, and then decreased to 104.98C at 8 wt. % MWCNTs. The TGA curves of pure PAN and PAN fibers with different weight percentages of graphene nanoplatelets and MWCNTs exhibited a four-step weight loss. In FTIR spectra, the intensities between 2,400 and 2,200 cm-1 for all samples of PAN having different weight percentages of MWCNTs and graphene nanoplatelets corresponded to the C≡N band for saturated nitrile groups.