This paper applies the linear instability analysis to the prediction of the boundary between transition and jetting regimes for forming disperse droplets using co-flowing immiscible liquid in coaxial microtubes. Results are compared to a flow regimes map developed based on numerical simulations covering wide range of parameters and liquids properties. The results are also compared to reported experimental measurements for co-flowing ionized water and PDMS oil in coaxial microtubes. Although captures the general trend, the linear instability analysis consistently under predicts and over predict the boundary between transition and jetting at low and high Ca_d, respectively. This is because the analysis neglects the secondary perturbations and the inertia of the disperse liquid. Nonetheless, the linear instability analysis is a nearly accurate predicative approach that is much faster than performing Computation Fluid Dynamic (CFD) simulations or conducting experiments.