In this paper, we propose a new test method to detect write disturbance fault (WDF) for magnetic RAM (MRAM). Furthermore, an adaptive diagnosis algorithm (ADA) is also introduced to identify and diagnose the WDF for MRAM. The proposed test method can evaluate process stability and uniformity. We also develop a built-in self-test (BIST) circuit that supports the proposed WDF diagnosis test method. A 1-Mb toggle MRAM prototype chip with the proposed BIST circuit has been designed and fabricated using a special 0.15-mu m CMOS technology. The BIST circuit overhead is only about 0.05% with respect to the 1-Mb MRAM. The test time is reduced by about 30% as compared with the test method without using the decision write mechanism. The chip measurement results show the efficiency of our proposed method.