Knowledge of the time course of penile erection is very important to understanding erection physiology. The changes in the intracavernosal pressure (ICP) and the different phases of an erection are pivotal to the ability to produce and maintain a rigid penile erection. This study investigated an objective and low-invasiveness method for identifying different erection phases based on an innovative ICP analysis technique. Blood infuses into the corpora cavernosa and causes the ICP to increase. The ICP usually exhibits tiny oscillations at the frequency of the heartbeat when it increases from diastole to systole. The characteristic oscillation amplitudes corresponding to the period when the full and rigid erection phases begin can be extracted by power spectral density analysis. The reliability and accuracy of the proposed method was verified by the Bland-Altman graphs indicating a good agreement with the existing method that compares the ICP with the arterial pressure. Moreover, all of the intraclass correlation coefficient values were close to 1.00, with the lower limit of the 95% confidence interval exceeding 0.75. The described novel objective and low-invasiveness method can therefore be used for identifying the full and rigid erection phases of the penis in urological investigations during different erection phases.