The very minimum chirp keying (VMCK) modulation possesses characteristics of an ultra narrow band (UNB). This paper uses a general analysis method of continuous phase modulation (CPM) to select a linear signal as the integral pulse, give the continual phase modulation express of VMCK, and construct a new kind of the VMCK transmitter structure. The normalization power spectral density formula of VMCK is then derived, with the power percentage bandwidth as an inspection criterion. It is proven that, for an orthogonal binary signal set, VMCK has a narrower main lobe compared with the minimum shift keying (MSK) modulation. Therefore better band width efficiency can be achieved with VMCK.