An asynchronous data segmentation/transmission method for synchronous Ethernet and a data structure used therein are disclosed. The asynchronous data segmentation/transmission method can segment and transmit asynchronous data such that the asynchronous data can be transmitted through a transmission frame for each transmission cycle. The method for synchronous Ethernet includes the steps of: a) comparing a size (L2) of asynchronous data to be transmitted with a size (L1) of transmission space vacant in an asynchronous frame part of a transmission frame, b) inserting the asynchronous data to be transmitted into the transmission space if the size (L2) of the asynchronous data is equal to or less than the size (L1) of the transmission space, and c), if the size (L2) of the asynchronous data is larger than the size (L1) of the transmission space, segmenting the asynchronous data to fit the size (L1) of the transmission space to insert the segmented asynchronous data thereto, and inserting remaining asynchronous data (L2-L1) into a next transmission frame. The transmission frame includes a More flag field for indicating asynchronous data segmentation/transmission.