Each parts of vehicles have possibility of fail occurrence, which potentially may cause field action on sold vehicles. Each manufacturers are spending astronomical amount of field action cost, and are required to decide correct field action level. Warranty trend analysis is essentially required for decision, which enables best field action level decision by projecting warranty frequency based on each conditions. In the long view, this satisfies customer, keep brand reputation and increase manufacturer"s income. Hereby, method and effect of warranty trend analysis & projection will be reviewed.