The research was conducted at the Indonesian Center for Rice Research, Sukamandi, Subang, West Java from August 2007 to August 2008. The research covered 150 ha of rice in lowland area with three rice crops in a year. The aim of the research was to study the population growth of the rice field rat in rice fields implementing such cropping pattern. Rat population sampling was done using Trap Barrier System (TBS) and Linear Trap Barrier System (LTBS) methods, where both systems were considered as physical rat control techniques. Seven TBS units with 25 m x 25 m dimension were set near the rice fields. LTBS unit was installed around perimeter of the entire rice field. The total length of LTBS was 10,000 m and equipped with 150 traps. The rice field rat population was observed by calculating the number of rats caught in TBS and LTBS traps. The total number of rats was counted every day during the rice planting season (RPS) from RPS-1 to RPS-3. The other variables observed were rice damages area due to rat, and rice yield. Results showed that rice field rats population increased significantly from the first crop season (RPS-1) to the following crop seasons (RPS-2 and RPS-3). Rice field rats caught in TBS increased from 224 rats in RPS-1 to 492 rats in RPS-2, and 677 rats in RPS-3. Similar trend was seen in the rat number caught in LTBS, which increased from 429 rats in RPS-1 to 1,423 rats in RPS-2 and 1,733 rats in RPS-3. The rice rat damaged area during the three planting seasons was categorized as light damaged, less than 20%, and the rice yields in each planting season were considered normal, between 5.53 ton/ha and 7.38 ton/ha.