Generally, DNA strand displacement is considered a second-order reaction, with its rate depends on the concentration of the invading strand when the concentration of the invading strand is higher than the substrate complex. While Genot et al. discovered the reaction rate is insensitive to the concentration of the invading strand with 14 nt toehold domains and 17 nt spacer domains, the overall displacement kinetic is approximately first-order. Besides that, Machinek et al. found the first-order rate constant fitting model is much better for strand displacement involving 10 nt toehold domains and proximal or central mismatch within the branch migration domain. Therefore, we considered two rate constant fitting models. These models assume that some of substrate complexes do not participate in the complete strand displacement. Each model contains two parameters, rate constant k and yield y. First-order rate constant fitting model as follows: