Persistent photoconductivity (PPC) has been investigated for Zn0.3Cd0.7Se mixed crystal. We found that local potential fluctuations induced by composition fluctuations are responsible for PPC observed here. However, the PPC is observed only above 70 K, in contrast to previous reported results on most materials which exhibit PPC effects predominantly at low temperatures. PPC relaxation shows stretched-exponential decay in temperature region 70 K<T<220 K. A PPC-related phase transition has been observed, in which the stored charge carriers experienced a transition from localized to delocalized states at temperature Tc(120 K). Stretched-exponential decay parameters as functions of temperature have been determined.