A theoretical model was developed for predicting the film thickness by spin coating on five poly(amic acid) solutions, PI2540, PI2545, PI2570, PI2571, and biphenyl dianhydride-p-phenyl diamine/n-methyl pyrrolidone/NMP. The solvent concentration, solvent diffusivity, and viscosity of poly(amic acid) solutions during spin coating were analyzed. The mechanism of polyimide film formation by spin coating based on the convective flow and solvent evaporation was discussed in this study. The ratio of the convective rate over evaporation rate is from 570 to 0.14 in the spin time of 0 to 60 s for PI2545. Hence, the ratio of the cumulative loss by convection over evaporation is 14. This result suggests that the convective flow by the centrifugal force plays a more dominant role in the coated film thickness than does the solvent evaporation for the studied poly(amic acid) solutions. The film thickness of the coated pyrometallic dianhydridel/4,4'-oxydianiline him decreasing from 2000 to 4500 rpm is in the reverse trend of the density of the poly(amic acid) solution. Hence, the film thickness from poly(amic acid) solution in n-methyl pyrrolidone depends largely on solution density, spin speed, and spin time.