In order to satisfy PM regulation in diesel passenger vehicle, it’s mandatory to install DPF (Diesel Particulate Filter) system within the vehicle. DPF system can store PM ingredient and it will be eliminated by oxidation reaction. But Ash ingredient within PM can not be eliminated during vehicle’s life cycle. Therefore we have to know how much ash will be accumulated in DPF. In this paper I suggest Ash Learning Logic for accurate detection of accumulated ash in DPF. From this logic, it’s possible to prevent abnormal detection of ash which may influence high fuel economy or DPF failure.