Recurrent aphthous ulceration (RAU) or recurrent aphthous stomatitis (RAS) and oral lichen planus (OLP) are the most common oral mucosal disease known to human beings. The immunological imbalance & psychological stress are considered to the most prime factors for Recurrent aphthous ulcer (RAU) & Oral lichen planus (OLP).Further the oxidative stress caused by nitric oxide has a definitive role in establishing the correlation between [NO] with RAU & OLP.The present study was undertaken to study the role of nitric oxide in human saliva & its diagnostic/prognostic role in RAU & OLP.The study was carried out at Government Dental college & Hospital,Aurangabad.Twenty cases with RAU,Fifteen with OLP & Thirty healthy control individual were included in the study. The clinically diagnosed known cases of RAU & OLP were included after taking detail case history. Under all aseptic precautions, fresh saliva was collected in sterile bulp.The sample was subjected to biochemical analysis of [NO]. The RAU cases were subdivided into major & minor and OLP cases were divided into erosive or non–erosive type after clinical & histological examinations. The salivary Nitric oxide levels were found to be increased significantly in RAU & OLP group when compared with controls. Further significantly increased levels have been observed in OLP group when compared with RAU group (P<0.001).The salivary nitric oxide levels were found to be increased significantly in minor RAU than major RAU and increased in erosive type of OLP than non–erosive type of OLP. Thus this parameter can be treated as a diagnostic tool for the diffential diagnosis of RAU& OLP