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　(sigma　=　23　similar　to　498　MPa)　and　dynamic　elastic　modulus　(E　=　3.5　similar　to　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　degrees　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.