Layered transition metal dichalcogenides which are part of the two dimensional materials family are experiencing rapidly growing interest owing to their diverse physical and optoelectronic properties. Large area controllable synthesis of these materials is required for transition from lab scale research to practical applications. In this work, we present a single step chemical vapor deposition process for large area monolayer growth of molybdenum selenide (MoSe2). We also demonstrate controllable thermal conversion from molybdenum selenide to molybdenum sulfide.