It's not something you can turn on or off but it depends on the method used to import the file.
To keep the mp3 an mp3 without any conversion, open the Project Audio Browser, and drag and drop your mp3 to the browser, or choose Audio File > Add Audio File.
Thanks for replying.
I'm doing same way with your advice, but the files are converted to AIFF format via any way...
Anyway, I guess I have to clean up my Logic by erasing pref files or something...
Thank you David.