ABSTRACT: First, entropy decrease in isolated system is proposed due to internal interactions. Second, we discuss entangled states and possible entropy decrease. Many entanglements are all interactions. The deterministic macroscopic entanglement is observed directly in 2021. Third, we propose that entropy decrease exists necessarily in self-assembly as isolated system. Fourth, we research various possible entropy decreases in condensed matter physics, which includes phase transformation from disorder uniformity to order state. Fifth, the solidification forms spontaneously an order structure, and it may be process of entropy decrease. Finally, we discuss the molecular motor and some other preconditions, in which entropy decreases possibly.