The pandemic COVID19 pneumonia has engulfed the entire world. Hematopoietic system can also be affected by COVID19. Thrombocytopenia at admission was prevalent, while late phase or delayed phase thrombocytopenia is obscure. This retrospective case series analyzed patients with COVID19 at the Union Hospital, Wuhan, China, from January 25th to March 9th, 2020. Analysis began on March 11th, 2020. COVID19 associated delayed phase thrombocytopenia was occurred in 11.8% percent of enrolled patients. The delayed phase thrombocytopenia in COVID19 is prone to develop in elderly patients or patients with low lymphocyte count on admission. The delayed-phase thrombocytopenia is significantly associated with increased length of hospital stay and higher ICU admission rate. Delayed phase nadir platelet counts demonstrated a high and significantly negative linear correlation with B cell percentages and serum IL 6 levels. We also presented bone marrow aspiration pathology of three patients with delayed phase thrombocytopenia, showing impaired maturation of megakaryocytes. We speculated that the delayed phase platelet destruction might be mediated by antibodies, and suggest immunoregulatory treatment in severe patients to improve outcomes. Besides, clinicians need to pay attention to the delayed phase thrombocytopenia especially at 3 to 4 weeks after symptom onset.