A method for measuring dielectric constant of body endermic tissues and body impedance based on the method of frequency digital sampling and for evaluating body composition is provided, inputting the method through the I/O interface of a microprocessor the measured body weight frequency signals, oscillating frequency signals related to dielectric constant of body endermic tissues and body impedance signals corresponding to non-fixed different frequencies, and calculating through the software of the microprocessor the body fat content, total body water, ratio between intracellular water and total body water. The body weight, body fat content, total body water and ratio between intracellular water and total body water are displayed on the display. A body composition monitor based on the above method, which includes a weighing sensor, a weighing signal processing circuit, and a display unit.