Vehicle　Automatic　Navigation　System　plays　an　important　role　in　intelligent　transportation　system,　the　model　of　optimum　route　selection　used　for　the　system　is　built　in　the　paper.　Considering　the　randomicity　of　frame　demand　and　supply　,　the　unblocked　reliability　of　the　road　is　used　to　dispose　the　uncertainty　of　the　congestion　happening.　Taking　advantage　of　historical　data　and　experience,　the　optimum　route　can　be　decided　based　on　the　analysis　of　unblocked　reliability　and　the　circuitous　length,　which　can　provide　the　reasonable　route　guidance　to　the　traveler　even　without　real　time　traffic　information.　©　2003　IEEE.