Abstract: A card reader device using a mobile and the method thereof is introduced. The mobile is adopted to supply electric power through audio output jack, and to transmit card data applied from the card reader device via microphone input jack. The card reader unit is connected to the audio jack of the mobile, and offers card data read by the card reader units via microphone input of the mobile. The card reader device comprises a contact type card reader unit to read cards with magnetic strips, and a contactless type card reader unit for contactless smart cards. The card reader device of the present invention enables and simplifies card transaction through mobile devices, and the reader is combinable with any mobiles with audio jack only.