The monitoring for measuring structural behavior has been advanced. Recently, many researchers have studied on the structural health monitoring using the GNSS (Global Navigation Satellite System). This paper presents temperature behavior for a Gwang-An bridge which is three span suspension bridge in Korea. The behavior of a middle span have analyzed on ambient temperature change. As the time passed from January to June in 2013, the vertical displacement was decreased and the temperature was increased more and more at the middle span. And the correlation analysis was performed between the temperature and the vertical displacement using the thermometer and GNSS. Also monthly changes of the temperature and natural frequency had been measured. And then the correlation analysis was performed between the temperature and natural frequency. As a result of the evaluation regarding thermal effect at the middle span, relationship between the temperature and natural frequency seemed to have trend of inverse proportion.