本文首先介紹CMARC3與MARC 21各自的機讀格式轉換發展概況，其次分別敘述CMARC3及MARC 21在定長欄部份（包括紀錄標示和資料代碼欄）的架構分布及特色，接著依CMARC3之欄位順序，逐欄比較CMARC3及MARC 21之定長欄內容，並依資料類型舉例說明。最後將定長欄特殊之轉換情形（如連續性出版品、兼具多種類型資源）加以釐清。CMARC3和MARC 21在定長欄的架構和使用上有很大的差異。MARC 21的定長欄在著錄前，須先決定待編文獻的資料類型，再依據資料類型填入資料內容代碼。CMARC3則可視資料類型需要，同時選用多個資料代碼欄。近年來，MARC 21因應新編目規範進行相關的修訂，中國機讀編目格式如何順應全媒體需求加以調整，也是值得關心的議題。

This article first introduces the general situation of machine readable form conversion, and then compares the contents of leader and datafields of CMARC3 and MARC 21 by material type. Finally, the transformation of the multi-type bibliographic records is discussed. It is concluded that there are many differences between the fixed fields of CMARC3 and MARC 21. When cataloging multi-type materials, catalogers have to decide the data type before using the fixed fields of MARC 21, while CMARC3 allows the users to use multiple fixed fields simultaneously. The revision of CMARC3 for adaption to the new media environment is also suggested.