This paper proposes a datagram delivery (class D service) architecture in an ATM-Internet, which is the network interconnecting ATM-LANs through the IWUs, Inter-Working Unit. We can provide a fast datagram delivery system through the following techniques. The datagram delivery to the destination terminal is performed by the datagram delivery server, so called CLS, which is located in the ATM-LAN where the destination terminal belongs to. Each CLS only manages the addresses for the terminals belonging to the corresponding ATM-LAN. The cells belonging to a certain datagram are transferred through a single (seamless) ATM connection from the source terminal to the CLS in the ATM-LAN where the destination terminal belongs to. The source terminal only resolves the access point address corresponding to the ATM-LAN where the destination terminal belongs to, when it submits the cells to the network to transfer the datagram to the corresponding destination terminal. The proposed datagram delivery architecture can be applied to the ATM-LAN system based on VPI routing architecture, easily. The number of the required ATM connections so as to provide datagram delivery through the proposed architecture is less than 1.0% of the ATM connections that the ATM-Internet can provide. Also, the required address space at UNI to provide datagram delivery are less than 1.0% of the UNI address space which is available to be used as an ATM connection identifier.