As mobile devices and mobile applications largelygrow, mobile data traffic increases at an unprecedented rate. How to alleviate mobile traffic congestion becomes a widelydiscussed issue. Because that most mobile data is not necessarilyused immediately (e.x. update applications, download a largesize video, etc.), these data are allowed to complete transmissionbefore delay-tolerant deadline. In this paper, we propose residualsize-based mobile data offloading for file transmission on mobiledevices. Our method periodically checks the residual sizeof the file. According to the residual size and delay-tolerantdeadline, our method decides how to use cellular networkand WLAN(Wireless Local Area Network) for file transmission. Besides, we also propose a cost function to investigatetrade-off between deadline and the fraction of the transferredsize using available WLAN. We compare our method withWifflers prediction-based offloading and On-the-spot offloading. The results show that, by the cost function, the trade-off isalways worth in our method compared to Wifflers predictionbasedoffloading and On-the-spot offloading. Our method is morelikely to guarantee file transmission completion before delay-tolerant deadline.