Abstract: A rotatable cross molecular beam machine discribed here consists of two pulsed beam units which are perpedicular to each other and are able to rotate synchronously about a certain axis along which a laser beam can cross with the molecular beams. The apparatus is equipped with a translationally movable ultrahigh vacuum mass spectrameter for time of flight (TOF)measurement. The detecting direction orients coplanarly with both molecular beams. The ions coming out of the mass filter are collected by a sensitive Dalytype detector. The apparatus is available for studies of formation of clusters, photolysis of molecules or clusters and collision of two particle beams. In this equipment the mass spectra of the clusters (CO₂)_n and (NH₃)_nformed in CO₂ and NH₃ supersonic expansion beams were measured respectively. The Angular Distribution (Lab) of CO₂⁺ arising from the collision of (CO₂)_n with Ar was also reported.