The increasing penetration of distributed generations (DGs) in the electrical system is causing a new system transient stability problem since most of DGs are characterized by low inertias and poor inherent damping. Measures such as application of storage unit and wind turbine crowbar protection have been proposed to enhance the transient performance of micro-grid. However, the increase in the number of micro-grid components also leads to changes in system critical clearing time (CCT) under fault conditions. This paper investigates the various features affecting the CCT of a micro-grid in an islanded mode. The result shows the traditional equation cannot be used to calculate the CCT and the wind turbine disconnection is the main reason causing the micro-grid collapse. The DG penetration level and the wind turbine crowbar protection insertion time can have significant impacts on the CCT value, and the CCT can be substantially increased by utilizing battery storage in the micro-grid.