This paper proposes a systematic approach to derive analytical and explicit certificates for transient stability analysis of inverter-based microgrids. We first derive a line-based model of microgrids with general mesh structure. Then by employing Lyapunov stability theory with a quadratic Lyapunov function, the region of attraction of any post-fault stable equilibrium point (EP) is estimated by a large domain which contains the well-known principle region inside, if a low-dimension convex LMI problem is feasible. Accordingly, it provides a very efficient and robust transient stability certificate which can be calculated off-line, easily, and very fast. Moreover, it can be applied to microgrids with any structure. Finally, numerous tests on a microgrid are then given to illustrate the effectiveness of the proposed approach.