In taekwondo, reaction time (RT) represents a pillar between success and failure. Some electroencephalogram (EEG) studies with taekwondo athletes have demonstrated that reaction time (RT) is associated with the spectral power (SP) of certain frequency bands of brain activation. In a previous pilot study, we observed the occurrence of positive correlations between the integral of the spectral power of the delta bands (0.5–4 Hz) with the RTs and simultaneously, non-significant trends of negative correlations between the integral of the spectral power of the alpha band (8.1 to 12 Hz) and the RTs, in electrodes of the frontal and temporal lobes. In this pilot study, the ratios of EEG signal SPs in the delta and alpha bands (delta/alpha) of one taekwondo athlete were measured during situations of response to simple visual stimuli. To do this, the volunteer performed 10 kicks against a punching tower, in response to visual stimuli with randomized times, with EEG electrodes. The kicks were analyzed in RT and SP, in different brain frequency bands (Delta and Alpha). Next, the SP values of the delta/alpha ratio, from the electrodes of the frontal and temporal lobes, obtained immediately before the kick executions, were correlated with the RT values obtained. The tendency of the data on the ratios of the spectral power integrals of the Delta/Alpha bands was to correlate positively with the reaction time (median rho of the frontal region: 0.72, 1st Qrt: 0.64, 3rd Qrt: 0.78, p = 0.02, 1st Qrt: 0.008, 3rd Qrt: 0.04; temporal region rho: 0.69, 1st Qrt: 0.26, 3rd Qrt: 0.90, p = 0.07, 1st Qrt: 0.001, 3rd Qrt: 0.33), indicating that the greater the activation in the delta band and simultaneously, the lower the activation in the alpha band, the longer the reaction time (RT).