For the enhancement of carbon nanofilaments (CNFs) formation density and the surface electrical properties, the source gas flow was intentionally manipulated as the cyclic on/off modulation of C(2)H(2)/H(2) flow in a thermal chemical vapor deposition system. CNFs formation density on the substrate and surface electrical conductivity of as-deposited substrate were investigated as a function of the cyclic modulation interval time for one cycle, namely the time for H(2) | C(2)H(2) flow (C(2)H(2) flow on) plus the time for H(2) flow (C(2)H(2) flow off). The optimal interval time for the enhancement of both CNFs formation density and the surface electrical conductivity was around 3.5 minutes. The cause for the variation in the characteristics of CNFs according to the interval time of the cyclic modulation process was discussed in association with the variation of the remaining time of pure hydrogen gas concentration.