For　traffic　networks　with　fixed　demand　and　heterogeneous　users　differentiated　by　value　of　time,　when　there　exists　nonlinear　relationship　between　travel　cost　and　travel　time,　the　time-based　and　cost-based　system　optimization　models　and　the　user　equilibrium　models　are　formulated,　respectively.　If　the　system　performance　and　user　equilibrium　conditions　are　both　measured　by　cost,　it　is　shown　that　system　optimum　can　be　decentralized　into　multi-class　user　equilibrium　by　anonymously　charging　users　on　all　links.　It　is　further　proved　that　such　tolling　scheme　doesn＇t　exist　if　the　system　performance　and　the　users＇　decision　criteria　are　based　on　time　unit.