Abstract The Large Optical/infrared Telescope (LOT) is a ground-based 12m diameter optical/infrared telescope, which is proposed to be built in the western part of China in the next decade. To select the best site which satisfies the construction and future operation of LOT, data monitoring and comparing are required for all the candidate sites. The comparison of most of the instruments was done at Xinglong Observatory. These instruments include weather station, all sky camera, sky background meter and differential image motion monitor (DIMM). This paper introduces the instruments used in LOT site monitoring and the instruments comparisons. The results show that the instrument is stable and the data uniformity of the identical instruments is good. This paper provides a fundamental description of LOT site monitoring.