Dental plaque is one of the etiologic factors in gingival and periodontal disease. Considering the uses of lemongrass oil the aim of the present study was to find out the antiplaque property in vitro. Pooled saliva was put into the tissue culture plates and incubated for 72 hours for plaque. Each of the 4 wells were treated differently with using disclosing agent, lemongrass oil mouthwash( 0.25 % and 0.5%), distilled water, glodent tooth paste slurry and chlorhexidine mouthwash. The results showed that the lemongrass oil mouthwash at both the concentration showed reduction in the plaque better than that of chlorhexidine. The glodent toothpaste slurry reduced the plaue to a lesser extent to that of both lemongrass oil and chlorhexidine mouthwash. The present study concluded that the lemongrass oil mouthwash can be used as an adjunct to the mechanical oral hygiene.