【Objective】The functions of the three genes XcfabF1,XcfabF2 and XcfabB, which were annotated as encodes putative 3-ketoacyl-ACP synthase in fatty acid synthesis in Xanthomonas campestris 8004 genome, were studied. 【Method】Three genes coloned into expression plasmid pBAD24M separately were transformed into the fabB and fabF temperature sensitive mutant CY242 and CY244 of Escherichia coli. The activity of XcFabF1, XcFabF2 and XcFabB in vitro was assayed by cell-free extracts. 【Result and conclusion】The genetic complementary revealed that XcfabB and XcfabF1 genes could restore the growth of mutant CY242 and CY244 respectively, but XcfabF2 could not restore the growth of mutant CY242 or CY244. The cell-free extracts of XcFabF1 and XcFabB could elongate octanoyl-ACP to longer acyl-ACP, while XcFabF2 could not catalyse the elongation in vitro assay. The above results demonstrate that XcfabB and XcfabF1 encode 3-ketoacyl-ACP sysnthase I and 3-ketoacyl-ACP sysnthase II respectively, but XcfabF2 is not involved in long chain fatty acid synthesis in X. Campestris.