A method of preparing a silicon-on-sapphire (SOS) substrate comprising the steps in the following order of: implanting ions into a monocrystalline silicon substrate or a monocrystalline silicon substrate having an oxide film thereon to form an ion-implanted layer therein; performing a surface activation treatment on a surface of a sapphire substrate and/or a surface of the ion-implanted monocrystalline silicon substrate or the ion-implanted monocrystalline silicon substrate having an oxide film thereon; bonding the monocrystalline silicon substrate or the monocrystalline silicon substrate having an oxide film thereon and the sapphire substrate to each other at 50°C or higher but not higher than 350°C, and then applying a heat treatment at 150°C or higher but not higher than 350°C to obtain a bonded body; embrittling an interface of the ion-implanted layer of the bonded body; and applying mechanical impact to the interface of the ion-implanted layer to split the bonded body along the interface, thereby transferring the monocrystalline silicon film to the sapphire substrate to form an SOS layer, wherein the step of embrittling the interface of the ion-implanted layer and/or the step of applying the mechanical impact to the interface of the ion-implanted layer is carried out with heating the bonded body at a temperature of 50°C or higher but not higher than 350°C, within a temperature range of 50°C higher or lower than a temperature in the step of bonding, and wherein the SOS substrate comprises the sapphire substrate and the monocrystalline silicon film on or above the sapphire substrate, and stress of the silicon film as measured by a Raman shift method has an absolute value of 2.5 × 10 8 Pa or less across an entire in-plane area of the SOS substrate. implanting ions into a monocrystalline silicon substrate or a monocrystalline silicon substrate having an oxide film thereon to form an ion-implanted layer therein; performing a surface activation treatment on a surface of a sapphire substrate and/or a surface of the ion-implanted monocrystalline silicon substrate or the ion-implanted monocrystalline silicon substrate having an oxide film thereon; bonding the monocrystalline silicon substrate or the monocrystalline silicon substrate having an oxide film thereon and the sapphire substrate to each other at 50°C or higher but not higher than 350°C, and then applying a heat treatment at 150°C or higher but not higher than 350°C to obtain a bonded body; embrittling an interface of the ion-implanted layer of the bonded body; and applying mechanical impact to the interface of the ion-implanted layer to split the bonded body along the interface, thereby transferring the monocrystalline silicon film to the sapphire substrate to form an SOS layer, wherein the step of embrittling the interface of the ion-implanted layer and/or the step of applying the mechanical impact to the interface of the ion-implanted layer is carried out with heating the bonded body at a temperature of 50°C or higher but not higher than 350°C, within a temperature range of 50°C higher or lower than a temperature in the step of bonding, and wherein the SOS substrate comprises the sapphire substrate and the monocrystalline silicon film on or above the sapphire substrate, and stress of the silicon film as measured by a Raman shift method has an absolute value of 2.5 × 10 8 Pa or less across an entire in-plane area of the SOS substrate. <EOS>