In vitro attachment were determined on a acrylic resin (Meliodent, Heraeus Kulzer GmbH and Co., Germany) and a Co-Cr alloy (CoCr-Modellguß-legierung, Degussa Dental, Germany) denture-base materials using the type strain of two types C. albicans (ATCC 26555 and Serotype B Netherland CBS 5983) and S. mutans (NCTC 8177). The results of assays showed that there was more attachment of two C. albicans to acrylic resin base material than to Co-Cr alloy base material, while there was more attachment of S. mutans to Co-Cr alloy than to acrylic resin. Attachment was prevented or reduced considerably in the presence of Thymus oil. In spite of this, Thymus oil exhibited the most pronounced inhibitory effect against both C. albicans, but it did not show inhibitory effect against S. mutans. Also, we have observed that the greater hydrophobicity of cells result in the greater attachment to some denture-base material surfaces. Howewer, we haven’t determined this relationship between hydrophobicity and attachment in all applications. Additionally, Thymus oil effected the attachment of microorganisms mentioned above