A method of accessing and processing a file stored in a sub-memory (135) of a portable terminal (Fig.1) including the sub-memory (135) and an internal memory (133) capable of storing mass storage data. The method includes storing a file system menu of the sub-memory in the internal memory (315) during a boot-up operation (313), displaying the file system menu when shifting into a file access mode for the sub-memory from a waiting mode, accessing file data selected from the file system menu and buffering the file data in the buffer of the internal memory (411,613) until the file data have a size corresponding to a predetermined upper limit value (413,615), processing the buffered file data (617), buffering data of the sub-memory in the buffer (423) until the data have a size corresponding to the upper limit value (427) when a quantity of buffered data becomes lower than a predetermined lower limit value (417) while processing the buffered file data, and repeating data processing and buffering procedures until a closing command is generated (419,429,625).