Correlation　of　network　delays　or　latencies　among　network　nodes　is　important　for　network　measurement.　Although　there　have　been　some　studies　on　this　subject,　few　have　studied　the　problem　thoroughly.　In　this　paper,　we　study　the　similarity　among　network　nodes　based　on　RTT.　First,　we　study　the　effects　of　path　length　and　load　on　RTT　and　conclude　that　they　may　affect　similarity.　Then,　we　study　the　effects　of　these　two　factors　on　similarity　respectively.　For　the　effects　of　path　length,　similarity　has　the　same　mutational　trend　as　the　common　path　length　and　the　inverse　mutational　trend　as　the　private　path　length.　For　load　combinations,　relationship　between　similarity　and　path　length　correlates　to　that　between　similarity　and　path　ratio　which　is　defined　as　the　ratio　of　the　common　path　length　to　the　longer　private　path　length.　And　our　study　shows　that　similarity　has　the　same　mutational　trend　as　the　path　ratio　with　the　ratio　1:1　being　the　inflection　point.　As　for　the　effects　of　path　load,　similarity　has　the　same　mutational　trend　as　the　common　path　load　and　the　inverse　mutational　trend　as　the　private　path　load　in　any　length　combination.　For　the　27　load　combinations　we　used,　in　a　length　combination,　relationship　between　similarity　and　path　load　correlates　to　that　between　similarity　and　load　ratio　which　is　defined　as　the　ratio　of　the　common　path　load　to　the　heavier　private　path　load.　And　our　study　shows　that　similarity　has　the　same　mutational　trend　as　the　load　ratio.　In　general,　the　more　the　two　source　nodes　have　in　common　along　the　paths　to　a　common　destination　node,　the　more　similar　the　RTTs　are　between　the　two　nodes　to　the　common　destination　node.　©　2012　ACADEMY　PUBLISHER.