In this paper, mobile lane departure warning systems (M-LDWS) are addressed. The proposed lane departure warning (LDW) detection algorithm has been successfully implemented on handheld devices with their build-in cameras. The users as long as download LDW program to their own mobile phones and the driving safety service will be enabled. M-LDWS is settled behind the windshield and the driver will be alerted when his car departs from the host lane. M-LDWS detects lane boundaries ahead the host vehicle and estimates the position within the host lane by the captured real-time image. It is qualified to be applied on highways, expressways and urban roadways in Taiwan. It also has been successfully verified in sunny, cloudy and rainy conditions for day and night. M-LDWS will be a novel killer application in consumer devices for driving assistance systems.