The conventional method for measuring the axial length of the eye with the envelope detected A-mode wave form is problematic because we cannot define distances which are less than about one wave length, and the points detected with simple minded voltage level are uncertain. We have recently studied a new method of measurement which solves these problems, and wish report on its theory and results of the experiments. This method is designed to obtain the high resolution and reliability at measuring of the distance using the technique of RF-signal processing. The received RF-signal of one scan line which we recognize on the B-mode tomogram is A/D converted and is processed by the computer through the following 3-stages. First, the signal is deconvoluted with the posterior echo of the lens, which is an accurate reference when the ultrasonic beam corresponds exactly to the axis of the eye for extracting the reflection series. Next, the specular echoes that correspond to the boundary of the tissues are extracted as nonstationary waves using the nonlinear prediction filter. This filter has separates the nonstationary waves from stationary background waves. Finally, we estimate the depth of the anterior chamber, the thickness of the lens, the depth of the vitreous body and the axial length of the eye from the intervals of the extracted impulse signals.