Abstract: ASN.1 is a standard definition and description language for abstract syntaxes.It offers a method of defining complex data types and specifying the values of these data types.ASN.1 is used as a means of defining and describing data structures by many OSI application standards,especially for defining the structures of various Application Protocol Data Units（APDU）.ASN.1 data types and basic encoding rules are introduced in this paper.The design and implementation of ASN.1 encoder/decoder in an MMS messaging system are discussed in detail.This encoder/decoder is used for the conversion between ASN.1 abstract syntax and transfer syntax of MMS Protocol Data Units（PDU）,helping to achieve MMS interoperability in a different machines and different operating systems environment.Finally,concrete improvement methods of raising encoding/decoding efficiency are also presented.