Datum transfer is usually carried out to satisfy requirements of fixture and operation in manufacturing process. Often such requirements are different than the functional requirements used when choosing tolerances during design. Reassignment and recalculation of tolerances are required to evaluate whether a machining scheme can reach the desired design specification. Since a planar feature are widely utilized in datum reference frame, this paper mainly focuses on the transfer of one datum plane to another parallel planesegment with round or polygonal shape by using 3D Tolerance- Maps. Firstly, a parametric model about two parallel planar features is proposed to describe the geometric properties and spatial relationships between a design datum and a machining datum. Then, based on a variation analysis of plane-to-plane datum transfer, a more elaborate transformation is utilized to relate T-Maps (Patent No. 6963824) with M-Map. Next, several rigid mathematical inequalities are presented to describe the boundaries of primitive and transformed T-Maps for further accumulation and comparison. The largest possible machining tolerances are determined by fitting the M-Map inside the design T-Map. Analytical relations between the machining and design tolerances are extracted from cross-sections through both T-Maps. Lastly, an example is illustrated to show feasibility of the method.