Red blood cells (RBCs) play a crucial role in delivering oxygen to the body tissues. During the 120 days of their lifespan, average RBCs would circulate for approximately 500,000 times and undergo repeated deformations in small blood capillaries and splenic cords. Increased RBC clearance in the spleen is considered as one of the direct consequences of reduced RBC deformability. On the other hand, deformability is also indirectly impacting on RBC functionality through its complex connections with underlying molecular mechanisms. With the aid of microfabrication and microfluidic, we are able to perform high-throughput single cell deformability measurement. Overall, we established RBC deformability as an important biomarker for several blood related real world problems such as malaria and blood storage lesion. The ultimate goal is through our quantitative assessment of population-wide single cell deformability, we could aid in the decision-making of various clinical scenarios including drug screening and blood transfusion. Malaria is the most deadly parasitic disease affecting hundred millions of people worldwide. Infected RBCs are found to be less deformable and therefore more susceptible to splenic RBC clearance. In this thesis, we identified several clinically used anti-malarial drugs that are capable of altering RBC deformability and ultimately modifying RBC retention in spleen. We also employed a rodent malarial model, confirming the important connection of RBC deformability with splenic RBC retention and consequently malarial anemia in vivo. Blood storage lesion is another important application of our work. Taking the advantage of device high-throughput, we profiled hundreds of single RBC deformability from a large population and identified subpopulations that are less deformable. These subpopulations also exhibited higher osmotic fragility and were therefore predicted to pose higher transfusion risk according clinical standard. Furthermore, a deformability based sorting device was also developed to filter the less deformable blood subpopulations, improving overall blood quality.