The　pores　introduced　by　the　inclusion　of　fibers　affect　the　mechanical　properties　of　engineering　cementitious　composite　(ECC).　In　this　paper,　the　effect　of　defoamer　on　the　mechanical　properties　of　ECC　was　investigated.　Different　contents　(0.05　wt.%,　0.10　wt.%,　0.15　wt.%,　and　0.20　wt.%)　of　defoamer　were　used　for　this　purpose.　The　results　showed　that　the　inclusion　of　defoamer　could　significantly　increase　the　compressive　strength,　first-crack　strength,　ultimate　tensile　strength,　and　tensile　strain　of　ECC.　A　dosage　of　0.15%　was　the　optimal　defoamer　content　for　enhancing　the　mechanical　properties　of　ECC,　as　further　increasing　defoamer　content　beyond　0.15%　compromised　the　efficiency　of　the　improvement.　The　inclusion　of　defoamer　reduced　the　total　number,　size,　and　volume　of　pores　in　ECC,　resulting　in　the　increase　in　the　fracture　toughness　of　matrix　and　fiber/matrix　interfacial　bond.　Based　on　the　micromechanics　theory,　the　inclusion　of　defoamer　increased　the　pseudo　strain　hardening(PSH)　indexes　that　govern　the　multi-cracking　behavior.　The　results　from　micromechanics　analysis　well　explained　the　trend　of　tensile　properties　of　ECC　with　different　defoamer　contents.　©　2022　Elsevier　Ltd