Purpose: The purpose of this study was to examine the effects of treadmill walking exercise on the BMD (Bone Mineral Density) and BMI (Body Mass Index) of middle-aged women in their forties. Method: A nonequivalent control group pretest-posttest design was used for this study. The experiment was conducted for 10 weeks from May 17 to July 25, 2004 with 29 middle-aged women in their forties, who were assigned to a walking exercise group (16) or a control group (13). The experimental group had not exercised regularly before participating in this program. The control group received no exercise treatment during the research period. The BMD and BMI were measured with peripheral dual energy x-ray absorptiometry and bioelectrical impidence analysis, respectively. $X^2-test$ and Mann-Whitney U test with the SPSS version 11.0 program were used to analyze the data. Result: Treadmill walking exercise was effective for middle-aged women in increasing BMD and decreasing BMI. Conclusion: The findings of this study indicate a need to develop walking exercise protocols for middle-aged women to help them achieve a healthy life.