This study investigated the causes of relinquishing the dogs adopted from animal shelters. The subjects were the adopters of public animal shelters in Taiwan in 2003 similar to 2005. Random sampling was conducted upon 10% of the population. This study collected data by telephone survey, and 1,185 calls were made, with 626 valid samples. According to this investigation, when being adopted, most of the animals were below one year old (71.7%; 449/626); among 626 dogs investigated, 229 were relinquished (36.6%; 229/626). They were mostly relinquished within half year after the adoption (45.4%; 104/229). The main cause of abandonment is "behavior" (31.9%; 73/229); "barking" is the most (72.6%; 53/73). Based on the results, the findings are as follows: 1) dogs below one year old are easily adopted; 2) "within half year" after the adoption is the critical period of abandon; 3) the dogs' behavior is the main cause of abandon. Among varied behaviors, barking is the major cause.