Though continuous development and progress have been made in the early diagnosis and treatment of cancer, it is still difficult to find a sensitive, accurate and minimally invasive biomarker for cancer diagnosis and treatment. MicroRNA (miRNA) is a class of non-coding small endogenous RNAs of 21-24 nucleotides in length. As a novel molecular biomarker, extracellular miRNA (ec-miRNA) has the potential to be a minimally invasive, highly sensitive and highly specific marker in cancer diagnosis. Many research achievements of ec-miRNA have been accumulated in recent years. In this paper, the origin, function and detection of ec-miRNA, its role in lung cancer diagnosis as a novel molecular biomarker, and some issues are reviewed.