This　paper　aimed　to　determine　the　softening　laws　and　fracture　toughness　of　slag-based　geopolymer　(SG)　concrete　and　mortar　(SGC　and　SGM)　as　compared　to　those　of　Portland　cement　(PC)　concrete　and　mortar　(PCC　and　PCM).　Using　three-point　bending　(TPB)　tests,　the　load　vs.　mid-span　displacement,　crack　mouth　opening　displacement,　and　crack　tip　opening　displacement　curves　(P-d,　P-CMOD,　and　P-CTOD　curves)　were　all　recorded.　Bilinear　softening　laws　of　the　PC　and　SG　series　were　determined　by　inverse　analysis.　Furthermore,　the　cohesive　toughness　was　predicted　using　an　analytical　fracture　model.　The　cohesive　toughness　obtained　by　experimental　study　was　consistent　with　that　predicted　by　analytical　method,　proving　the　correctness　of　the　tension　softening　law　obtained　from　inverse　analysis.　In　addition,　both　initial　and　unstable　fracture　toughness　values　of　SG　mortar　were　lower　than　those　of　PC　mortar　given　the　same　compressive　strength.　Moreover,　the　initial　fracture　toughness　of　SG　concrete　was　generally　lower　than　that　of　PC　concrete,　whereas　the　unstable　fracture　toughness　exhibited　an　opposite　trend.　©　2020　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.