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A theoretical quantum key distribution scheme using EPR pairs is presented. This scheme is efficient in that it uses all EPR pairs in distributing the key except those chosen for checking eavesdroppers. The high capacity is achieved because each EPR pair carries 2 bits of key code.Since languages become the tool for communication, the desire and need to transmit secret messages from one person to another begin. Then human have the cryptography -an art to transmit information so that it is unintelligible and therefore useless to those who are not meant to have access to it. The most important classic cryptographic scheme is public-key crypto-system [1], its safety relies on the high complexity of the underlying mathematical problems, for instance the factorization of large numbers. But with the development of the quantum computation(QC), especially the Shor's algorithm for factoring big numbers, the systems once seemingly unbroken in practice will be agressed easily. Now in the information community, the safety of transmission of secret information is becoming more and more concerned. One essential theme of secure communication is to distribute secret keys between senders and receivers. Quantum mechanics, one of the greatest discovery of the 20th century, has now entered the field of cryptography: if the key distribution makes use of quantum states, an eavesdropper can not measure them without disturbing them. The principle of the quantum mechanics can help to make the key distribution secure. Upto now, there have already been several quantum key distribution(QKD) schemes: BB84 protocol [2], the EPR scheme [3,4],B92 [5], the 4+2 protocol [6], the six-state protocol [7], the Goldenberg/Vaidman scheme [8], Koashi/Imoto scheme [9], and the recent Cabello protocol [10] and so on.Experimental research on QKD is progressing very fast, for instance the optical-fiber experiment of BB84 and B92 protocols have been realized upto 48 km [11] and experiment in free space of B92 scheme has been achieved over 1 km distance [12], and very recently upto 1.6 km during daylight [13] .

A protocol for quantum secure direct communication using blocks of EPR pairs is proposed. A set of ordered N EPR pairs is used as a data block for sending secret message directly. The ordered N EPR set is divided into two particle sequences, a checking sequence and a message-coding sequence. After transmitting the checking sequence, the two parties of communication check eavesdropping by measuring a fraction of particles randomly chosen, with random choice of two sets of measuring bases. After insuring the security of the quantum channel , the sender, Alice encodes the secret message directly on the message-coding sequence and send them to Bob. By combining the checking and message-coding sequences together, Bob is able to read out the encoded messages directly. The scheme is secure because an eavesdropper cannot get both sequences simultaneously. We also discuss issues in a noisy channel.