In cooperative communication systems, hierarchical modulation is used to increase systems robustness and to send different information flows simultaneously. In cooperative communication systems which use hierarchical modulation, error propagation is the most important problem that prevents achieving full diversity gain. Thresholds that depend on the signal to noise ratio (SNR) value between the source and the relay can be used to mitigate error propagation. If the instantaneous SNR between the source and the relay is lower than the first threshold, the relay does not transmit. If the SNR is between the first and the second thresholds, the relay demodulates and forwards only the primary bits. If the SNR is higher than the second threshold, the relay demodulates and forwards both the primary and the secondary bits. Mitigating error propagation and achieving full diversity for both primary (high priority) and secondary (low priority) bits depend on setting the thresholds at the relay properly. In this work, the first and the second threshold values are determined so that full diversity gains are attained for both primary and secondary bits. Analytical and simulation results are provided to verify the analysis.