The paper proposes a more formalized definition of UML 2.0 Activity Diagram semantics. A subset of activity diagram constructs relevant for business process modeling is considered. The semantics definition is based on the original token flow methodology, but a more constructive approach is used. The Activity Diagram Virtual machine is defined by means of a metamodel, with operations defined by a mix of pseudocode and OCL pre- and postconditions. A formal procedure is described which builds the virtual machine for any activity diagram. The relatively complicated original token movement rules in control nodes and edges are combined into paths from an action to action. A new approach is the use of different (push and pull) engines, which move tokens along the paths. Pull engines are used for paths containing join nodes, where the movement of several tokens must be coordinated. The proposed virtual machine approach makes the activity semantics definition more transparent where the token movement can be easily traced. However, the main benefit of the approach is the possibility to use the defined virtual machine as a basis for UML activity diagram based workflow or simulation engine.