Taiwan's climate is marine tropical. The northern part of the island has a rainy season that lasts from January through late March during the northeast monsoon, and experiences hot, humid weather from June through September. Thus, road’s maintenance is a difficult issue due to its long-season rains as well as high groundwater levels. In the mean time, the current roads are generally weaker in most suburban areas due to those soft soil bases uncompacted in construction. Those aspects explain the low performance of suburban roads in northern Taiwan. Therefore, this research takes the Yilan county, located in the northeast part of the island and well-known as a rainy county, as a study area where major suburban roads are selected to take asphalt concrete specimens for testing. The major purpose is to investigate the time effect of asphalt pavements for those of specimens with a period time of three years. A total number of 329 asphalt concrete specimens are characterized as Class I specimens, 305 of which were taken in 2007, as well as Class II of 24 taken in 2010. First, as for the Class I specimens, the Indirect Tensile Strength test (ITS) is conducted, together with the Control Chart as a filter, and then the remaining 253 specimens come to the average ITS value of 15.55 kgf/cm2. Secondly, as for the Class II specimens of 24, the Film Stripping (FS) tests are conducted to obtain the Index of Retained Strength (IRS) values, from which the basis of ITS comparisons is found. Thus, the different average ITS value, 19.16% in three years, is obtained. The result also indicates that the annual reduction of ITS value for an area similar to the study area may be taken as approximately 11% (33.14/3 %) and 4.7% (13.98/3 %) for the conditions of high traffic flow and low traffic flow, respectively. It may also address the common effect of traffic and environment on moisture damage in asphalt pavements.