In the last decade, the 2-4 week extended range forecast (ERF) has become an important research area for operational developments of both weather forecast and climate prediction. Propagation of intraseasonal oscillation especially MJO (30-60 d oscillation) is a fairly effective method to be used for ERF at present. Internationally, preliminary achievements have been made in years of operational experimentation on weatherclimate forecasting. In this review, the intraseasonal oscillation (ISO) as theory basis for ERF is firstly briefly introduced, including basic characteristics of ISO and interactions between ISO and weatherclimate in middle latitudes. Then, developments of predictability, methods and operational application of ERF are reviewed. And signals and relative predictability of ISO for ERF of China are further explored by taking Meiyu over the YangtzeHuaihe Basin as an example. Finally, the developing trend of ERF is described by pointing out some problems in dynamical models.