This paper newly proposes an adaptive symbol timing synchronization method based on maximum likelihood estimation for the realization of multi-mode & multi-service software radio, which is possible to search accurately for the optimum symbol timing from digitally over-sampled signal without the rate conversion circuit when the system clock rate is non-integer times of the symbol rate for the received system. Thus we can realize multi-mode & multi-service software radio by minimum number of signal processing components like AID converter. Finally, we suppose the triple-mode receiver for ITS, which are Japanese Personal Handy-phone System (PHS), Electric Toll Collection (ETC) and Global Positioning System (GPS), and evaluate the jitter performance and bit error rate performance by computer simulation.