The photophoretic force in the free-molecular regime has been calculated for a spherical particle using the Lorenz–Mie solution to the electromagnetic field within the particle. The temperature distribution on the surface of the suspended particle is calculated using a finite difference method. The effect of the complex refractive index m=n+ik and the normalized size parameter defined as α=2πa/λ on the photophoretic force and particle velocity are also examined. We show that for a 1 solar constant illumination, the photophoretic forces might be as high as 20% of the weight of the particles considered.