Background: Oral epithelium forms a physical, chemical and immunological barrier against invading microorganisms. The chemical barrier mainly comprises of anti-microbial peptides (AMPs) like cathelicidins. Transcription of cathelicidin anti-microbial peptide (CAMP) gene is induced by binding of the bioactive form of vitamin D to vitamin D receptor. Thus, an increase in vitamin D should significantly improve the chemical barrier function by increasing AMPs. This study explored vitamin D in primary prevention of chronic periodontitis by reinforcing the epithelial barrier by increased action of cathelicidin. This was a single center, randomized, two-way, crossover design study. The aim of this study was to assess the effect of vitamin D administration on levels of cathelicidin LL-37 in gingival crevicular fluid (GCF) of chronic periodontitis patients. Methodology: Forty vitamin D insufficient subjects with chronic periodontitis were divided into two groups. GCF sampling was done a day after the periodontal examination. Following this, all the patients were subjected to scaling and root planning (SRP). Test group patients were prescribed oral dose of 2000 IU vitamin D daily for a period of three months. It was followed by a six week washout period. Then the groups were subjected to crossover and the same treatment sequence was followed. Cathelicidin LL-37 level was estimated using Enzyme Linked Immunosorbent Assay (ELISA) method. Results: Vitamin D administration increased the levels of cathelicidin in periodontal diseases. Cathelicidin levels in GCF increase after vitamin D intake. Conclusion: Periodontal diseases might be prevented by reinforcing the epithelial barrier (chemical barrier) by vitamin D induced production of AMPs.