The Material Extrusion (ME) technique in 3D printing facilitates the production of thermoplastic materials with diverse cellular or lattice-like infill geometries, enabling the creation of lightweight, high-performance materials. This research investigates the influence of infill geometry and relative density on the fracture toughness and effective fracture energy of 3D-printed thermoplastics produced through ME. Polylactic acid (PLA) was used as the model material and various unit cell geometries and densities are explored. First tensile tests are conducted to determine in-plane elastic moduli in two orthogonal directions, considering unit cell anisotropy, which are employed in effective fracture energy calculations. Second, compact tension fracture tests are conducted to quantitatively characterize the crack growth characteristics in two orthogonal directions. In both tests, digital image correlation method was used to measure full field strain distributions. The study unveils three significant findings. Firstly, there is a power scaling relationship between Elastic Modulus and relative density for all unit cells, and Kagome exhibits the largest correlation constant. Triangle and Kagome unit cells exhibit negligible orthotropic responses in perpendicular directions for cell densities. Secondly, a similar scaling-law governs the relationship between nondimensional fracture toughness and unit cell density, with minor variations depending on unit cell type. The correlation factor displays slight dependence on infill geometry within the examined relative density range, maintaining a value of 3. Thirdly, considering the dissipation of inelastic fracture energy, both Triangle and Kagome unit cell geometries demonstrate an order of magnitude higher effective fracture energy (i.e., crack growth resistance) compared to the square unit cell. This is associated with toughening mechanisms that emerge from the deflection and branching of cracks at the crack tip. Therefore, relying solely on elastic fracture toughness proves insufficient for evaluating the fracture resistance of 3D-printed thermoplastics with varied infill geometries and densities. These novel findings highlight the substantial disparity between fracture toughness and effective fracture energy in assessing the fracture resistance of 3D-printed thermoplastics with lattice-like infill. Additionally, the proposed scaling laws provide predictive insights into the fracture toughness of polymeric materials produced through the ME method.