Introduction: Although there are many methods in malaria diagnoses e.g., quatitative buffy coat (QBC), rapid diagnosis tests (RDTs), serological tests and molecular diagnosis methods such as PCR, but microscopy still remains the gold standard for malaria diagnosis. Estimation of malaria parasite density can be carried out by using assumed white blood cells (WBC) and red blood cells (RBC) counts. Objective: The aims of this study were to determine malaria parasite densities calculated by assumed WBC and RBC counts; and to compare their reliability with absolute WBC and RBC counts. Methods: The clinical presentations and laboratory findings of specimens collected from 512 uncomplicated falciparum and vivax malaria patients admitted to Hospital for Tropical Diseases, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand were utilized and analysed for estimation of malaria parasite densities by using different formulas. Results: Parasite densities calculated by WHO recommended assumed WBC of 8,000 /µL, and assumed RBC counts of 4.7x106-6.1x106 /µL and 4.2x106-5.4x106 /µL for males and females respectively led to overestimation, and resulted in low reliability when compared to the absolute WBC and RBC counts. Parasite densities calculated by assumed WBC of 5,900/µL in thick blood; by assumed RBC of 4.8x106/µL for males and 4.3x106/μL for females in thin blood film respectively gave more precise estimation. Conclusion: Assumed WBC and RBC counts for calculating malaria parasite densities have to be adjusted to use in Thailand for more precise estimation. Parasite densities calculated by assumed WBC and RBC used in other malaria endemic countries might need further re-evaluation.