Building　performance　optimisation　(BPO)　has　been　intensively　explored　due　to　its　potential　in　improving　building　performance　and　design　efficiency.　However,　studies　on　its　application　in　supporting　early-stage　design　decisions　are　rather　limited,　causing　its　actual　effectiveness　to　remain　questionable.　This　paper　addresses　the　problem　through　a　design　experiment,　where　participants　designed　an　office　building　for　energy　efficiency,　using　first　the　conventional　method　and　then　BPO.　They　also　evaluated　others＇　design　outcomes　and　completed　a　questionnaire　about　the　experiment　afterwards.　The　results　showed　that　BPO　is　effective　in　supporting　early-stage　architectural　design　even　for　novices.　It　turned　manual　designs　that　are　often　averagely　energy-efficient　in　the　solution　space　into　top　designs　without　significantly　affecting　other　design　merits.　The　average　energy　use　intensity　(EUI)　decreased　by　8.5　%　in　this　experiment.　However,　it　showed　a　narrow　advantage　(0.5　%　on　average)　over　evaluating　a　series　of　design　samples　and　no　clear　superiority　over　less-automated　supporting　methods　with　more　involvement　of　designers,　as　its　effectiveness　is　limited　by　technical　difficulties　in　implementing　certain　variables　strongly　correlated　with　energy　efficiency.　In　addition,　while　the　majority　of　participants　acknowledge　the　effectiveness　of　BPO　and　the　acceptance　of　its　design　outcomes,　manual　designs　were　still　preferred　by　more,　mostly　for　better　aesthetics　and　better　alignment　of　design　concepts,　implying　the　necessity　of　designer＇s　interventions　before　using　BPO　outcomes　in　design　practice.　The　useability　of　BPO　also　needs　to　be　improved　by　shortening　the　calculation　time　and　lowering　the　difficulties　in　building　desirable　parametric　models.