Background: Oral candidiasis, which is one of the most common diseases of the mucous membrane, is carried in the mouth of about 17-75% of healthy population. And 51-74% of this healthy population is asymptomatic. However, oral candidiasis is found in 50-65% of the individuals with dentures, is found in 67% of elders over 75 years old and is found in 85% of the patients with oral cancer. The most common causative organism is C.albicans with 43.7%, less commonly C.glabrata and C.tropicalis. According to some researches, the occurrence of the oral candidiasis has been increasing continuously for last 20-30 years, and has strong tendency to consistently increase for the 21st century. Methods: Total 50 patients were participated in this study, including; the patients with full denture from Department of Prosthodontics of The Dental Center of Ulaanbaatar, and patients with evident symptoms of candidiasis from Department of Neck and Head Surgery and Department of Radiation Therapy of National Cancer Center of Mongolia. The study was carried out by descriptive study type, based on the patients’ questionnaire survey, physical examination and laboratory tests. Results: The oral candidiasis causing pathogen Candida was found in 100% of following patients including; the patients with full denture and the patients having head and neck radiation therapy. For the patients with full denture, Candida albicans was the most commonly implicated organism with 53%, Candida tropicalis 21% and Candida krusei 11%. For the patients with radiation therapy for head and neck cancer, most common pathogen was Candida albicans with 64% and Candida tropicalis with 7%. Conclusions Main causative pathogens for the patients with full denture are C.albicans (53%), С.tropicalis (21%), and for the patients with radiotherapy for head and neck cancer is C. albicans (64%). 95% of the participants who were diagnosed with oral candidiasis were resistant to Colistin and 100% sensitive to Nystatin and Miconazole.