The temperature sensor in board house of GPS radio-collar is for monitoring the ambient temperature of collared animals. We assumed that the effect of body heat of an animal on the temperature sensor would result in biased temperature reading and evaluated the reading by using GPS3300S collars (Lotek Wireless Inc.). We examined the bias by comparing the temperatures readings among collar sensors and two temperature loggers, i.e. an internal logger wrapped by artificial heaters inside the fur of sika deer (Cervus nippon) on the board house for representing the animal body, and an external logger for measuring ambient temperature. We managed two loggers working synchronically with the sensor and regarded the difference between loggers as animal body-ambient-difference, and the temperature difference between the external logger and the sensor as sensor bias. We confirmed that the body heat affected the sensors positively with body-ambient-difference. We designed the collar for reducing sensor bias by creating spacer between the board house and the animal neck by one or two plates for insulating body heat and combining an aluminum membrane pasted on the house and the mid-plate for reflecting body heat. Increasing the spacer width and pasting reflecting membrane reduced sensor bias in both single- and double-spacer designs. A 12-mm single-spacer design with one house reflecting membrane worked best and bias decreased to 2.1°C at 70°C body-ambient-difference.