Estimating channel state information (CSI) in the fast fading conditions is very challenging. In this paper, a simple structure for cooperative diversity in decode-and-forward mode is examined, in which the transmitter, the relay and the receiver do not require to know CSI. In this scheme, transmission of information is done in a two phase process. In the first phase, differential modulated signals are radiated from the source. After decoding the received signal in the relay side, the source and the relay collectively send information using differential space-time codes. The optimum power allocation in a sense of minimizing pairwise error probability (PEP) in high SNR scenarios has been obtained using an analytical method. An interesting property of the optimum point is that it is independent of the channel statistics and the position of the relay, which improves its feasibility in the future communication systems. Numerical simulations verify the analytical results.