Primary thyroid lymphoma (PTL) is a rare disease which responds well to rituximab-based chemotherapy. Here, we describe a case who was diagnosed through core needle biopsy as having diffuse large B-cell lymphoma in the right lobe of thyroid gland. Positron emission tomography computed tomography (PET-CT) revealed no other foci of hot spots, so PTL was considered. She was treated with rituximab plus bendamustine for three cycles, and color Doppler ultrasound revealed significant reduction of blood flow signals in the tumor but no significant decrease of its size (<25% extent). Then, the chemotherapy regimen was adjusted to rituximab, cyclophosphamide, vincristine, prednisone (R-COP), and complete remission was noted on ultrasound and PET-CT after three cycles of R-COP treatment. This case is reported to tell that color Doppler ultrasound, in addition to PET-CT, is useful to evaluate chemotherapeutic effect on PTLs.