A test for the Arm Length Stabilization (ALS) is underway at the caltech 40m prototype interferometer. The experimental setup has been completed for a single arm and we have started examining the performance of the ALS scheme. The scheme employs a frequency-doubled auxiliary laser to sense the arm cavity motion prior to full lock acquisition. It enables us to hold the arm length at a desired value to ease the complexity of the successive lock acquisition steps. A successful lock of the ALS has been demonstrated for the single arm, resulting in a residual displacement of 200 pm in rms.