The　ambient　vibration　measurement　is　a　kind　of　output　data-only　dynamic　testing　where　the　traffics　and　winds　are　used　as　agents　responsible　for　natural　or　environmental　excitation.　Therefore　an　experimental　modal　analysis　procedure　for　ambient　vibration　testing　will　need　to　base　itself　on　output-only　data.　The　modal　analysis　involving　output-only　measurements　presents　a　challenge　that　requires　the　use　of　special　modal　identification　technique,　which　can　deal　with　very　small　magnitude　of　ambient　vibration　contaminated　by　noise.　Two　complementary　modal　analysis　methods　are　implemented.　They　are　rather　simple　peak　picking　(PP)　method　in　frequency　domain　and　more　advanced　stochastic　subspace　identification　(SSI)　method　in　time　domain.　This　paper　presents　the　application　of　ambient　vibration　testing　and　experimental　modal　analysis　on　large　civil　engineering　structures.　A　15　storey　reinforced　concrete　shear　core　building　and　a　concrete　filled　steel　tubular　arch　bridge　have　been　chosen　as　two　case　studies.　The　results　have　shown　that　both　techniques　can　identify　the　frequencies　effectively.　The　stochastic　subspace　identification　technique　can　detect　frequencies　that　may　possibly　be　missed　by　the　peak　picking　method　and　gives　a　more　reasonable　mode　shapes　in　most　cases.