A low-dielectric constant thin film prepared from a silsesquioxane polymer matrix as a precursor and a method for preparing the same which comprises preparing silsesquioxane sol by adding a stereoisomer of a multi reactive cyclosiloxane to an alkoxysilane are provided. The low-dielectric-constant thin film retains a stable film state even at a curing temperature of −500° C. without being decomposed, a very uniform surface property with a low surface modulus, and a superior coatability as to be coatable smoothly with no crack even with a thickness of 500 nm or larger.