A system and method for monitoring the stability of a hybrid powertrain system includes monitoring the system and its subsystems output response signals to its inputs to identify the system operating status. For example, for a hybrid powertrain system equipped with an electrically variable transmission (EVT), three signals are monitored. They are the two electrical motor speeds representing the direct subsystem response to their feedback control inputs and the EVT output speed representing the entire system's response. A stability monitoring system includes a mean determination module that determines a mean signal of the signals and an oscillation determination module that determines a signal oscillation signal based on the instant signal and the mean signal. The monitoring system also includes a signal mean crossing determination module that determines the signal crossing its mean signal and the oscillation peak detection, storage and comparison modules that determines system instability indicators for the control system to take corrective actions.