A new algorithm for phase reconstruction from a single carrier-frequency interferogram without phase unwrapping is proposed. The algorithm directly derives two real-valued partial derivatives of the desired phase from the interferogram. Through direct integration of the two derivatives, the desired phase with high resolution is retrieved. The algorithm needs no phase unwrapping process, so the phase error or distortion brought by unwrapping operation is avoided. The algorithmn is efficient and insensitive to the nonuniformity of the intensity distribution of the interferogram and the shape of the domain boundaries. The feasibility of the algorithm is demonstrated by both numerical simulation and experiment. The method is effective for real-time measurements of dynamical processes.