Considered to be extensions of the Hipparcos reference system,PPMX and UCAC3 are two of the most important astrometric catalogs released in current years.Extensive analyses of these two large catalogs have been made in order to determine the local and overall systematic biases.The regional and magnitude dependent differences in stellar position and proper motion are comparable to random errors and are even larger in the northern hemisphere.The global orientation bias vector （∈） between the two systems is also significant (up to 17 mas),which shows the overall differences of the PPMX and UCAC3 catalogs and their reference systems.On the other hand,the term for the global rotation vector ω is small (tenths of mas per year):it is reasonable to believe that the PPMX and UCAC3 reference frames do not rotate with respect to each other.Because of plate dependent and field-to-field errors in the UCAC3 catalog,we suggest that positions and proper motions of UCAC3 stars in the northern hemisphere (δ ＞ -20°) should be used with caution.