微正则系综(microcanonical ensemble)是由许多具有相同能量，粒子数，体积的体系的集合。它是统计力学系综的一种。其配分函数是在能量上的能态密度。

微正则系综是个简并度下的正则系综，正则系综可以被分开进入子系综，每个子系综被对应到可能的能量值且自身为另一个个的微正则系综。人们可以用正则系综去计算和导出相关的物理量，如熵或能量密度，所需要的是由微正则系综中的微正则配分函数Ω来运行此计算。

例如熵(entropy )：

其中是玻尔兹曼常数(Boltzmann constant)。

In statistical mechanics, a microcanonical ensemble is the statistical ensemble that is used to represent the possible states of a mechanical system which has an exactly specified total energy. The system is assumed to be isolated in the sense that the system cannot exchange energy or particles with its environment, so that (by conservation of energy) the energy of the system remains exactly known as time goes on. The system's energy, composition, volume, and shape are kept the same in all possible states of the system.