Side-by-side laminar flow formation in a microchannel has been applied for many micro-reactor applications, such as chemical synthesis and biochemical analysis, mixing, and particle classification. However, the gravity influence on laminar flow has not been much investigated because its influence has been considered to be very small due to largeness of surface effects. In this work, quantitative investigation on gravity influence on laminar flow was carried out using computational fluid dynamics (CFD) simulation. As a result, it was shown that the gravity influence is proportional to density difference, channel size, and average residence time, and inversely proportional to viscosity, in the condition that the microchannel cross-section is square and interfacial tension and diffusion are small. Degree of the gravity influence on laminar flow can be estimated easily using this relationship. Experiment using serum for biochemical analysis was carried out and verified this characteristic. The results in this work are useful especially for biochemical analysis, and additionally, mixing, chemical analysis, and synthesis.