This paper investigates the effect of porosity and unit cell size variation in topology optimized (TOP) and selective laser melting (SLM) fabricated Ti6Al4V lattice structure on the mechanical properties including compressive strength, failure mechanism and dynamic elastic modulus. Meanwhile, mathematical relations between mechanical properties and geometric parameters are obtained based on Gibson-Ashby model. The results show that both ultimate compressive strength (σ = 23∼498 MPa) and dynamic elastic modulus (E = 3.5∼55.47 GPa) of TOP lattice structures gradually decrease with the increase in porosity and unit cell size. The analysis combining experimental and numerical results indicates that TOP lattice structures are elastic-brittle porous material and have two failure mechanisms. The numerical predicted stress-strain curves are compared with the experimental ones. The numerical models incorporating the Johnson-Cook damage model could predict the slip direction of 45° failure band and ultimate compressive strength. Classical Gibson-Ashby model was used to predict the relation between relative density and mechanical properties of lattice structures. The exponential factors (n) of fitted models are obviously affected by unit cell size, which are determined by the number of unit cells in compressive test and SLM manufacturability in dynamic elastic modulus test. A 3D Modulus-Density-Unit Cell Size model is innovatively proposed, which can provide theoretical basis of tailoring orthopedic implant filled with functional gradient TOP lattice structures.