Abstract: The use of feed-forward back propagation neural networks to predict ionospheric F₂ layer critical frequency, f₀F₂, 24 h ahead, have been examined. The data we used are from Wuhan ionospheric station, China, and Hobart ionospheric station, Australia. The data period is from 1970 to 1990 at Wuhan and from 1962 to 1990 at Hobart. The five day's measurements of f₀F₂ before the day that need forecast are reduced to five coefficients. The inputs used for the BP neural network are the coefficients, the solar 10.7cm flux index, and the outputs are the day's 24 h observed f₀F₂ data. The trained net then can forecast f₀F₂ 24 h advance. The result indicates the predicted f₀F₂ using NN has good agreement with observed data. Comparison with IRI model suggests that NN method is more accurate than IRI. In addition, the error analysis indicates that predicted f₀F₂'s Root-Mean-Square Error (RMSE) is smaller in Hobart than in Wuhan, smaller in low solar activity than in high solar activity, smaller in winter and summer than in spring and autumn. In conclusion, using neural network to predict ionospheric parameters is a feasible method.