An　optimization　of　oxide　aperture　diameter　to　match　the　light　emission　aperture　(the　central　defect　region)　in　the　photonic　crystal　vertical-cavity　surface-emitting　laser　(PhC-VCSEL)　is　presented　in　order　to　obtain　low　threshold　current　and　high　single-fundamental-mode　output　of　the　device.　A　3-D　finite-difference　time-domain　method　has　been　applied　to　simulate　the　mode　field　distribution　and　quality　factors　(Q　factors)　of　fundamental　and　high　order　transverse　modes　which　correspond　to　the　mode　loss　of　the　PhC-VCSEL　cavity.　Single-fundamental-mode　operation　of　PhC-VCSEL　with　1.7-mW　output　power　and　low　threshold　current　of　0.7-mA　is　demonstrated　after　oxide　aperture　optimization.