We have performed precise measurements of the density of supercooled liquid silicon (l-Si) in the temperature range of 1530-1800 K using an electromagnetic levitation (EML) technique with static magnetic fields. We also performed first-principles molecular dynamics simulation (FPMD) of supercooled l-Si. The observed density of the supercooled l-Si and the FPMD results show good agreement in the temperature range of 1530-1800 K. The structure of the supercooled l-Si also showed good agreement between the experimental measurements and FPMD simulations. Based on these results, we discuss nucleation in supercooled l-Si and also the existence of a density maximum in the supercooled l-Si, which is well known in water at 4 degrees C.