In this paper, a detailed diagnostic of an ac glowto-arc discharge transition is presented. The behavior of the temporal evolution of voltage and current discharges is studied under an atmosphere of helium-methane. In order to determine the rotational temperature, an optical emissionspectroscopy analysis from glow-to-arc regime discharge is carried out, where the OH and C 2 bands, respectively, which are situated at 306.357 nm (A 2 Σ + , ν = 0 → X 2 Π, ν = 0) and 516.52 nm (d 3 Π g , ν = 0 → a 3 Π u , ν = 0), are used. Several metallic atomic lines were used to calculate the electron temperature. The principal discharge parameters, such as temperature and electron density in the arc regime, are determined. As an application of this glow-arc discharge, the synthesis of carbon nanofibers is reported.Index Terms-Carbon nanofibers (CNFs), glow-arc, highfrequency discharge, swan band, UV-OH band.