A method of arranging data protection in a telecommunication system including a first mobile communication network, a second mobile communication network, and a mobile station supporting both of the mobile communication networks is disclosed. The method includes ciphering traffic between the mobile station and the first mobile communication network using a first cipher key; calculating a second cipher key to be used for ciphering traffic between the mobile station and the second mobile communication network in the first mobile communication network when the mobile station operates in the first mobile communication network; transmitting information necessary for calculating the second cipher key from the first mobile communication network to the mobile station when the mobile station operates in the first mobile communication network; and calculating the second cipher key at the mobile station to be used for ciphering traffic between the mobile station and the second mobile communication network.