Multi-loop feedback control has attracted considerable attention due to its simplicity and ease of implementation. 1 In order to simplify the cumbersome analysis of a multiloop circuit, the signal flow graph representation should be used rather than our familiar nodal analysis methods. This paper presents a criterion from the well-established Mason's formula for multi-loop oscillator in terms of signal flow graphs. The multi-loop oscillator circuit based on two operational transresistance amplifiers (OTRAs) is used as an example.