Isothermal holding of an Al-19 wt%Si alloy containing 0.02%P has been carried out at 620°C to determine the rate of Ostwald ripening of primary Si crystals in Al melt. It is shown that the average radius of these crystals during isothermal holding can be expressed by an equation; R³−R₀³=kt, where a constant k is determined to be 1.6×10⁻¹² cm³/s. When the specimens are solidified at a constant cooling rate of ΔT⁄Δt, both solidification growth and Ostwald ripening take place simultaneously. This can be approximated as repetitions of quenching from T_n to T_n−ΔT followed by isothermal holding for the period Δt. The change of the average radius of primary Si crystals during solidification at constant cooling rate has been numerically analyzed by separating Ostwald ripening during isothermal holding from solidification growth during quenching. It is shown that contribution of Ostwald ripening increases with decreasing cooling rate and/or with increasing number of primary Si crystals formed at the onset of solidification. The number of primary Si crystals after complete solidification is primarily dependent on their number at the onset of solidification, and the effect of Ostwald ripening is of minor importance. These results obtained on primary Si crystals are also discussed in comparison with those on primary dendrites.