Abstract: In order to examine the resistance level and the potential resistance mechanism of Capsella bursa-pastoris to florasulam, six populations from wheat fields at Jinghai, Wuqing, Baodi and Jizhou in Tianjin were collected. Whole-plant bioassay was conducted to investigate the florasulam resistance levels, and acetolactate synthase (ALS) gene fragments were amplified, sequenced and compared between the resistant and susceptible populations. The results showed that the six populations were highly resistant to florasulam, and the resistance indexes were between 11.4 and 47.2. These six resistant populations were found to contain one amino acid mutation of proline (CCT) to serine (TCT) at the codon position 197 in the ALS gene. This mutation in the ALS gene might be one of main reasons for the resistance to florasulam in Capsella bursa-pastoris populations.