It is well known that omega phase has a significant influence on the precipitation of alpha phase in metastable beta titanium alloys. However, the assistance of the transformation strain induced by the formation of omega phase to the nucleation of alpha phase has not well been addressed. In this work, the precipitation of alpha phase with the assistance of the beta -> omega transformation was investigated in terms of the crystallographic orientation relationships and the accommodation of the transformation strains. The analyses of transformation strains indicate that alpha phase originates from the beta phase, but not from the omega phase due to the large strain of the omega -> alpha transformation. This corresponds to the experimental TEM/HRTEM observations in which alpha phase nucleates nearby the omega/beta interfaces and are separated with omega phase by the distorted beta matrix. The lattice distortion of beta matrix induced by the beta -> omega transformation can be well accommodated by the precipitation of alpha phase, and the vicinities of omega/beta interfaces are the favorable nucleation sites for alpha phase. This work provides new information on the omega-assisted nucleation of alpha phase in metastable beta titanium alloys.