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Power combining with array configuration is an effective method for high output power in oscillators with resonant tunneling diodes (RTDs) in subterahertz and terahertz range. In this paper, output power and stability are analyzed for coupled oscillator array of RTDs. The coupled van der Pol equations are derived from the equivalent circuit for two-element oscillator arrays at first, and the condition in which only the fundamental oscillation mode stably oscillates is shown. The oscillation frequency of the array is single due to the mutual injection locking even if the individual frequencies of oscillator elements are different. Minimum difference between the individual frequencies to keep the mutual injection locking is analyzed. Experimental results are explained with theory. The analysis is extended to multielement oscillator arrays, and characteristics of oscillation modes, stability, combined output power, and locking range of the frequency difference between the elements are obtained. Theoretical combined output power increases with element number in proportion roughly to square of the element number if the frequency difference between the elements is small, and decreases with frequency difference. Different array configurations for stable and high-power operation are also discussed briefly.