This study aimed at determining the current knowledge, attitude, and practices (KAP) of layer and pig farmers to antimicrobial usage and its consequences. Description of such KAP could provide insights useful for promoting the rational use of antimicrobials in livestock. From May 2014 to February 2016, a survey involving 251 respondents in Chiang Mai, Lamphun, and Chonburi provinces, Thailand, was conducted by using a validated questionnaire. More than half (51.0%) of the respondents incorrectly believed that antimicrobial drug efficacy could not be reduced by using sub-recommended dosages, 61.2% had misconceptions about non-therapeutic antimicrobial use, and 66.9% inaccurately felt that antimicrobials were also effective for virus and fungi. Over half (50.6-55.2%) did not see the need to follow instructions or advice of veterinarians. Moreover, only 10.4% regularly relied on responses to drugs sensitivity tests when evaluating the effectiveness of antimicrobials. Overall, assessment of KAP regarding antimicrobial usage indicated that the majority of respondents had low levels of knowledge of antimicrobials, neutral rather than positive attitudes, and employed poor practices in the use of antimicrobials. The results indicate improvements in KAP could be helpful in developing more effective interventions by farmers, reduce antibiotic usage, and slow the growth of antimicrobial resistance.