A series of in-plane quasi-static tensile and compressive tests are carried out on a Fiberglass/Phenolic hexagonal cell honeycomb core to characterize the uniaxial in-plane responses of the honeycomb core under large displacement. Test results show that the in-plane behavior of the honeycomb core is, in general, non-linear and anisotropic. Through the analysis of test results, ribbon fracture and node bond failures are recognized as the primary causes of honeycomb core collapse when the core is loaded along the ribbon and transverse directions, respectively. To study the in-plane behavior numerically, a 3D non-linear finite element model (FEM) with large displacements of the repetitive unit cell is employed in which the cell walls, node bond adhesive layers, and adhesive fillets at the intersections of the cell walls are modeled based on the measured geometry of a commercial Fiberglass/Phenolic honeycomb core. FEM results show the node bond adhesive and fillet region have significant effects on the in-plane mechanical properties of the honeycomb core including the core strength and tangent stiffness. Good agreement is observed between the model predictions and test results, particularly under tensile loading. Copyright 2017. Used by the Society of the Advancement of Material and Process Engineering with permission.