In order to fabricate a flexible composite of large dielectric constant, the preparation of dielectric fillers employing single-walled carbon nanotubes (SWNT) passivated with alkyl pyrenes having different alkyl chain lengths and their mixing with a polymer matrix were investigated. When the lengths of alkyl chains (n) of alkyl pyrene for SWNT passivation were varied from 0 to 4, dielectric constant of the composite were almost identical (K < 25). However, further increase regarding the length of alkyl chains in alkyl pyrene (n=12) dramatically increased the dielectric constant of the epoxy composite (K > 200) without loss of flexibility, despite the concentration of passivated SWNTs being as low as 0.04 wt% in the composite. Especially, highly suppressed dielectric loss was observed for the passivated SWNT/epoxy composite with alkyl pyrene containing an alkyl chain length of 12, which made it an ideal candidate for the embedded flexible capacitor prerequisite to flexible electronics. These dielectric behaviors were attributed to an effective debundling and partially aligned dispersion of SWNTs in the matrix, when alkyl chains with optimum length were employed in the alkyl pyrenes.