The influence of negative plasma triangularity on the n=1 (n is the toroidal mode number) tearing mode (TM) stability has been numerically investigated, with results compared to that of the positive triangularity counterpart. By matching the safety factor profile for a series of toroidal equilibria, several important plasma parameters, including the triangularity, the plasma equilibrium pressure, the plasma resistivity as well as the toroidal rotation, have been varied. As a key finding, the TM is generally more unstable in the negative triangularity plasmas as compared to the positive triangularity counterpart. The fundamental reason for this difference is the lack of favorable average curvature stabilization in negative triangularity configurations. Direct comparison of the Mercier index corroborates this conclusion. The plasma toroidal flow generally stabilizes the TM in plasmas with both negative and positive triangularities. The flow stabilization is however weaker in the case of negative triangularity with finite plasma pressure.