About 90 percent of dry hay is packed in the most countries which is provided as rectangular cubic bales by baler in Iran. The compression of alfalfa bales strongly depends on the yield maturity, cutting time and its moisture content. Therefore, the driver should continually control the value of bales density proportional to its moisture content. Otherwise, the bales with high density and moisture content cause mould growe, while the unusual light bales will increase the traortation and storing expense. Therefore, construction the system for adjusting the density of ales roportional to amount of their moisture content will be useful. In this regard Mousavifar designed and fabricated a device for a special verity of alfalfa at certain range of moisture content. In the present study, redesign and fabricating the same system for several kinds of variety, different cut and various moisture content range was considered. As well user friendly, simplicity and save expense were considered. The designed system included alfalfa moisture sensor, hydraulic system, control system, baler’s end plates pressure controller springs displacement gauge, and mechanism for changing the pressure of these springs to achieve a certain density. The moisture sensor was designed using ultrasonic wave which was consisted of an ultra-sound transducer (40 k Hz), a wave guide pipe with a ten-centimeter interior diameter and 30-centimeter length. Wave guide pipe was located vertically on the latest hay bales, and the produced wave by the micro control AT Mega 64 was sent by the ultra-sound sender which located in the wave guide pipe. The wave was received by the ultra-sound receiver just after contacting the bales. The degree of received waves is depended to the temperature, variety and moisture content of the hay. So a temperature sensor was used. The moisture sensor was calibrated with coefficient of determination 0.98 (R 2 =0.98) in range of 13 to 25 percent (w. b) moisture content of alfalfa. The, hydraulic system consist of olenoid operated directional valves, a pressure reducing valve, and a hydraulic cylinder. A linear potentiometer was used to measure the springs displacement in which indicated baler end plates position. The fabricated automatic control system of hay bales was evaluated in field at two cut harvesting in various yield moisture content. The results showed no significant difference ( p 5%) between bales density which provided by automatic control system and desired density. Also, performance of fabricated device showed no difference between cutting yield. Keyword : Hay moisture, Hay density, moisture sensor, Ultra-sound wave, baler