The temperature, humidity, air composition and pH value are very important parameters in the granary that keep the grain safely store and good quality. Nowadays only temperature and moisture are used in most granary monitor and control system. Air composition, pH and insect pest are not monitored because classic industrial personal computer or microcomputer are not able to handle these real-time multiply tasks at the same time. So it is necessary to design a monitor and alarm system to monitor these parameters in the granary to minimize or curtail damage or loss. Applying the advanced embedded technology and sensor technology, a set of granary temperature and humidity monitor and alarm system is designed based on the ARM. Temperature sensor, humidity sensor, air composition sensor and pH value sensor are selected to monitor the parameters in the granary. Based on the ARM microprocessor LPC2290, the hardware system consists of serial communication module, LCD display module and alarm module. Based on the embedded operation system, the software completes the allocation design and testing of the tasks and realizes the functions such as pulse frequency signal acquisition, analog signal A/D convert, serial digital signal communication, alarm in different grades and LCD display. All the parameters can be real-time displaying on the LCD screen. The system meets the requirement of the monitor and alarm functions in the granary.