Round-trip　time　(RTT)　is　defined　as　the　round-trip　delay　of　a　data　packet　between　a　source　and　a　destination　node　and　is　a　very　important　metric　for　measuring　the　performance　of　network　applications.　The　measurement　and　estimation　of　RTTs　has　received　a　great　deal　of　attention　in　network　measurement　research　over　the　years.　An　interesting　question　is　how　RTTs　from　different　source　nodes　to　a　common　destination　node　are　correlated,　which　describes　to　what　extent　the　RTTs　exhibit　the　same　changing　trend.　Such　correlation　is　useful　since　it　would　make　it　feasible　to　only　make　some　nodes　perform　RTT　measurement　while　some　others　estimating　RTTs　based　on　the　correlation,　thus　reducing　the　overhead　incurred　by　RTT　measurement.　In　this　paper,　we　perform　a　quantitative　analysis　on　the　correlation　of　RTTs,　in　particular,　how　network　topological　properties　can　affect　such　correlation.　Based　on　experiment　data,　our　analysis　shows　that　(1)　correlations　are　not　much　different　for　path　length　combinations　that　only　differ　in　the　length　of　the　shorter　private　path　but　will　become　the　weakest　in　statistics　when　the　lengths　of　the　two　private　paths　become　the　same;　(2)　correlations　are　not　much　different　for　path　length　combinations　that　have　the　same　path　length　ratio　(which　is　defined　as　the　ratio　of　the　length　of　the　common　path　to　that　of　the　longer　private　path);　and　(3)　correlation　increases　along　with　path　length　ratio　with　ratio　1/1　being　the　inflection　point.　The　results　from　our　analysis　of　RTT　correlation　can　be　applied　not　only　to　RTT　measurement　and　estimation,　but　also　to　the　deployment　or　selection　of　measurement　nodes　in　a　general　network　measurement　infrastructure.