As　the　road　traffic　especially　the　freight　transport　growing,　traffic　congestion,　traffic　accident,　noise　and　environmental　problems　become　more　and　more　seriously.　But　these　costs　are　not　all　paid　by　users　but　society,　and　also　the　roads＇　constructions　and　maintenances　are　invested　by　the　government.　It　is　to　say　there　impacts　are　externalized　on　the　society.　So　many　researchers　postulate　the　＇user　pay＇　principle　for　internalizing　external　cost　and　charging　the　user　of　transport　infrastructures.　This　paper　develops　a　methodology　to　evaluate　the　social　costs　including　internal　costs　and　external　costs　of　roads.　Social　costs　is　composed　of　internal　costs　and　external　costs,　therein　internal　costs　contains　time　costs　and　operation　costs,　and　external　cost　includes　maintenance　costs,　CO2　cost,　accident　cost　and　noise　costs.　Road　pricing　is　to　internalize　the　external　cost,　which　is　paid　by　the　government.　So　it　is　important　to　evaluate　the　social　costs　especially　the　external　cost　of　roads.　Here　an　evaluation　model　is　made,　and　then　the　road　network　data　in　Spain　are　used　to　validate　it.　First　it　is　necessary　to　divide　all　the　roads　by　the　number　of　lanes　and　then　choose　three　dimensions　(the　Flow　of　Roads,　the　Proportion　of　HGV　and　the　Space　Speed)　as　three　factors　to　regroup　all　types　of　roads　with　different　lanes　in　Spain.　K-meaning　method　is　used　to　cluster　each　type　of　roads,　for　every　type　we　choose　two　typical　clustering　that　one　is　called　the　representative　clustering,　and　the　other　is　highlighted　clustering　to　do　the　analysis.　Mathematical　software　is　used　to　calculate　the　social　cost　of　four　kinds　of　vehicles:　Car,　HGV　(Heavy　Goods　Vehicles),　LGV　(Large　Goods　Vehicles)　and　Bus.　Finally　this　research　get　the　conclusion　that　the　Spanish　government　spends　lots　of　money　on　the　maintenance　of　Spanish　roads,　but　not　so　much　users　are　using　those　roads,　the　cost　for　the　HGV,　LGV　and　BUS　are　much　higher　than　the　cost　of　Car　especially　the　maintenance　cost　and　the　maintenance　cost　for　HGV　is　very　high.　Certainly　the　result　can　give　some　suggestions　to　the　policy　of　roads　pricing　in　Spain.　Copyright　ASCE　2009.