Abstract: A neuroid BP-type three-layer mapping model is used for monthly rainfall forecasting with 1946－1985 Nanjing monthly precipitation records as basic sequences and the model has the form i×j=8×3.k=1 By steadily modifying the weighing coefficient, long-range monthly forecasts for June through December 1986 are constructed and month-to-month predictions are made based on the records of the preceeding months of the year, with mean absolute error reaching 6.07 and 5. 73mm, respectively. Additionally, with a set of four monthly initial valuesi; June through September, 1994, neuroid forecasting is indicating the same drought result in Nanjing durulg the summer, an outcome that is in good agreement with the observation.