Klebseilla pneumoniae possesses many virulence factors and survival strategies to persist and overcome host defenses; one of these strategies is biofilm formation. Therefore, the aims of this study was to determine the antibacterial and antibiofilm effect of Rosmarinus officinelis L. essential oil (EO) and its effect on the genes encoding of fimbrial adhesions. The antimicrobial activity was investigated by MIC. The ability to form biofilm as well as inhibition of initial cell attachment and biofilm formation was performed. PCR was carried out to detect fimH-1 and mrkD genes of type 1 and type 3 fimbrial adhesions at different time of incubation. The study revealed that MIC value of EO was 104 μg/ml on 24 (83%) of isolates, 93% of them produced biofilm. Fifty percent reduction in biofilm formation was observed in 10% of isolates at concentration 104 μg/ml and increased to 45% when used 1.5×104 μg/ml of EO. PCR product of fimH-1 was detected at 24 h but absence at 0 and 4 h while mrkD product found in all incubation time. In conclusion, Rosemary EO had antibacterial and antibiofilm activity against Klebsiella pneumoniae. Moreover, it affected the type 1 fimbriae at gene level probably by mutation during initial attachment of biofilm formation.