We propose THz oscillators using resonant tunneling diodes (RTDs) and slot antennas with stub-shaped reflectors. The stub-shaped reflectors are composed of metal-insulator-metal layer structure, and are put at the edges of slot antenna to obtain high reflection of electromagnetic field. Oscillation frequency and output power were analyzed theoretically including all the parasitic elements in RTD and three-dimensional electromagnetic simulation of the actual antenna structure. Fundamental oscillation up to 2.4 THz and output power of 70 muW at 1 THz are expected theoretically in GalnAs/AlAs RTDs with optimized device size, which are much improved values compared with the previous theoretical and experimental results without stub structures.