Three dimensional graphene-carbon nanotube networks (3D-GC) have attracted great interests due to their superior thermal, optical, and hydrogen storage properties. In our work, the in-plane mechanical properties of nanoporous 3D-GC with different diameters of the joint carbon nanotube (CNT) and porosity have been studied. During in-plane tension, the fracture of 3D-GC first initiates at the heptagonal defects of the junctions between graphene sheets and CNTs where large tensile residual stress is observed. The in-plane tensile strength of 3D-GC decreases with the increasing of CNT parameter and porosity, and the tensile modulus is mainly determined by the porosity. Although the fracture strain decreases with the CNT diameter, it increases with the porosity. Compared to the nanoporous graphene, 3D-GC has larger in-plane tensile strength and fracture strain due to the additional support of CNTs. However, the in-plane tensile modulus of 3D-GC is usually smaller than that of the nanoporous graphene due to the wrinkled configuration of 3D-GC. By considering the stress concentration and additional support of CNTs, a theoretical model is proposed which can describe the molecular dynamics simulation results well.