Abstract: The propagation of whistler-mode waves in the ionosphere is assumed to be the ducted propagation. In other words, the waves will be presumed as travelling along magnetic field lines. An analytic solution of the whistler dispersion equation is obtained in terms of the electron density models of the ionosphere the transverse gradient of the electron density required for guiding of whistler-mode waves along a magnetic field line, and an empirical formula for the magnetic field lines of 1980's IGRF (n = 8). In this paper, the electron density enhancement factor is defined as N_c/N_g-l (the N_c is the electron density at the centre of duct; the N_g is the background electron density at the latitude that is what the centre of duct lies). Thus, the integral mean value of the enhancement factor required and the path latitude (i.e. the exit point latitude of whistlers) can be analytically determined by a set of observed values: N_mF2 and h_mF2 of the ionosphere, and D (whistler dispersion). Using the data of D, N_mF2 and h_mF2 in Hainan Island of China, The above evaluation and come to the following conclusions are made. (1) The integral mean value of the enhancement factor required for ducting of low-latitude whistlers ranges from 7% to 31%. (2) The whistler dispersion Dhas a positive correlation with N_mF2; and a negative correlation with h_mF2 when the parth latitude φ₉₀≤10.5° (IGRF, n = 8). Conversely, if (φ₉₀>12° (IGRF, n=8) Dhas a positive correlation with h_mF2. It is found that 94.5% of the whistlers observed at Sanya (18.24°N, 109.5°E; geomag. lat. 7.04°N; IGRF, n = 8, lat. 9.64°N) of Hainan Island have their path latitudes φ₉₀<10.5°. Therefore, Dshould have a negative correlation with h_mF2, which is agreeable to observations.