Depending　on　the　randomness　of　traffic　demand　and　supply　analysis,　the　paper　presents　the　concept　of　unblocked　reliability　and　the　evaluation　methods　for　road　component　and　network　system.　Taking　the　unblocked　reliability　of　the　road　system　as　optimization　objective,　the　improvement　optimization　model　of　urban　transportation　network　is　built.　Through　the　component　importance　analysis　and　genetic　algorithm,　the　solution　can　be　found　effectively.　In　the　end　of　the　paper,　an　example　is　given　to　illuminate　the　practicability　and　maneuverability　of　the　suggested　probability　assessment　index　and　optimum　methodology　for　the　urban　transportation　network.