Introduction: Thrombocytopenia is a common symptom of malaria, and typically occurs more frequently in Falciparam malaria. It appears that P. vivax does trigger thrombocytopenia, contrary to popular belief. Therefore, patients with low platelets and fever ought to be considered for malaria, though B12 deficiency does not always accompany malaria cases. Methods: This study was conducted at indoor patients at the NIMS hospital to investigate the relationship between vitamin B12 with thrombocytopenia associated with dengue fever and malaria. Patients were investigated for routine investigations such as CBC, ESR, RFT, RBS, LFT, PBF, VIT B12 LEVEL, Dengue profile and MP card test. In out of 125 patients,75 patients have dengue fever, and 50 patients have malaria fever. Result: In dengue fever out of 75 patients, 61 patients (81%) had thrombocytopenia with vitamin B12 deficiency (B12 level <100 pg/l). In malaria out of 50 patients, 28 patients (56%) have thrombocytopenia with vitamin B12 level (B12 level >300 pg/l), 22 patients (44%) have thrombocytopenia with vitamin B12 level normal (B12 level 201-300). Vitamin B12 level < 190 pg/ml was found in 47(94%) patients with severe thrombocytopenia while 32 number (74%) patients with mild thrombocytopenia. In group with B12<190 pg/ l need of SDP transfusion was significantly high i.e; (115.13±42.08) in comparison to other groups, as well as the recovery time of platelets to 20000/µl threshold, was found to be high in B12 <190 pg/l group (42.60±8.89 days) as compared to other groups. Conclusion: Using platelet analysis by PBF, the results of this study were obtained. There is no clumping of platelets, and the platelets show the normal size, color, and shape.