The trends of CH4, H2O, HF, and HCl mixing ratios in the stratosphere and lower mesosphere for the 10 years were obtained from HALOE observations. The characteristic features in the trends and the potential mechanisms for those trends are discussed. In the tropical lower stratosphere near 20 km, CH4 mixing ratio increases continuously after the minimum between late 1993 and early 1994. The growing rate decreases for the recent several years and is consistent with the tropospheric observation. However, in the upper stratosphere near 50 km, CH4 decreased until October 1996 and recovered to the 1993 level in late 1997. CH4 has been decreasing again in the upper stratosphere since 1997. The mixing ratios of HCl and HF are discussed in relation with concentrations of the tropospheric CFCs. Total abundance of HCl and HF between the 48 and 60 km levels is closely related with that of tropospheric CFCs by the lag of 4-5 years. To understand the chemical mechanisms for the above-mentioned trends the University of Illinois at Urbana-Champagne 2-D model was used. Concentrations for the several chemical species obtained by the steady-state version of the model were compared with the climatological values from HALOE. Several model runs were done to reproduce the upper stratospheric decrease of CH4 with temperature and wind fields from UKMO data for the period of 1992 to 1997. The flux of the minor gases from the earth surface was specified based on the UNEP scenarios and other observed data. The model experiments show that increasing CH4 and CFCs in the troposphere play a role in determining the increasing trend in the lower stratosphere and the decreasing trend in the upper stratosphere of CH4, although the model results underestimate the upper-stratospheric CH4 decrease. The role of chlorine and fluorine from tropospheric CFCs in influencing the variations of CH4, HCl, HF and H2O is discussed.