We　restudy　the　master-equation　approach　to　aggregation　in　freeway　traffic　based　on　the　theory　of　birth-death　process,　in　which　the　clustering　behaviour　in　one-lane　freeway　traffic　model　is　investigated.　The　transition　probabilities　for　the　jump　processes　are　reconstructed　by　using　Greenshields＇　model,　and　the　equation　of　the　mean　size　of　the　cluster　at　any　time　t　is　derived　from　the　birth-death　equation.　Numerical　experiments　show　the　clustering　behaviours　varying　with　time　very　well.