We have designed a near field measurement system operating at 200 GHz which has a dynamic range of 60 dB. The system has a short term and long term phase variation less than 0.5° and 2°, respectively. On averaging 128 times at each measurement point by using the averaging function of the network anaylzer, the short term phase variation decreases to less than 0.2°. The phase variation mentioned above is observed with the testing probe pointing to the center of the antenna under test. The contours of the near field obtained shows that the field is highly circular as expected from the theoretical predication. We have shown that the system can distinguish the quality of the corrugated feed horn by near field measurement.