This　article　presents　an　optimal　placement　and　sizing　of　distribution　static　compensator　(DSTATCOM)　in　radial　distribution　networks　as　multi-objective　optimization　with　the　objective　of　power　loss　reduction,　voltage　profile　improvement　and　network　reliability　considering　different　variable　demands　using　whale　optimization　algorithm　(WOA).　The　WOA　is　inspired　by　the　hunting　behavior　of　humpback　whales.　The　optimal　location　and　size　of　DSTATCOM　are　considered　as　decision　variables　that　are　determined　using　the　WOA.　Network　reliability　is　defined　as　energy　not　supplied　(ENS)　of　the　customers　caused　by　line　outages.　The　proposed　method　is　implemented　on　IEEE　33-bus　and　Ahvaz　59-bus　distribution　networks.　The　optimal　placement　and　sizing　of　DSTATCOM　are　investigated　considering　different　residential,　commercial　and　industrial　loads,　and　also　the　effect　of　incorporating　reliability　is　evaluated　on　power　loss,　minimum　voltage　and　ENS.　The　simulation　results　show　that　the　power　loss　and　ENS　are　declined　and　also　voltage　profile　is　improved　by　determining　the　location　and　size　of　DSTATCOM　in　the　networks　in　different　loading　conditions　using　the　WOA.　The　obtained　results　demonstrate　that　considering　reliability　further　reduces　the　ENS　and　more　customer　demand　is　met.　The　power　loss　decreases　21.60%,　30.71%,　and　22.03%　for　residential,　commercial　and　industrial　loads　in　the　33-bus　network　and　26.86%,　31.94%,　and　24.39%　for　the　69-bus　network,　respectively　compared　to　base　networks.　Moreover,　the　performance　of　the　WOA　is　compared　with　previous　studies　and　the　results　indicate　the　superiority　of　the　proposed　method.　©　2013　IEEE.