An experimental study was conducted to investigate deflagration-to-detonation (DDT) transition of propane-oxygen mixtures in a smooth tube. Two types of DDT process were observed, which is induced by a localized explosion originating near the turbulent flame or by the precursor shock wave. Based on the criteria of self-initiation and autoignition limit, the critical turbulent flame burning velocity and amplification factor are estimated. The DDT induction distance, evaluated from the combustion and pressure wave front, is linearly dependent on the amplification factor under the specified initial conditions of reactant mixtures.