The effect of thermophoresis on the particle deposition on a cooled cylinderin non-isothermal laminar gas ow has been studied using Direct NumericalSimulations (DNS). Simulations where thermophoresis have been taken intoaccount for different Stokes numbers and particle-to-gas thermal conductivityratios, Λ, have been performed at Reynolds number Re = 380. In additionreference cases, simulations where thermophoresis have not been taken intoaccount, have been performed both for isothermal and non-isothermal owfor Re = 20 and Re = 380.The ratio between the front side particle impaction efficiency in the non-isothermal reference case and the isothermal reference case for the smallestStokes numbers considered was expected to be proportional to the ratio ofthe free stream temperature and the cylinder temperature, according to an-analytical considerations. The simulations for Re = 20 was in good agreementwith this relation, but for Re = 380 the front side particle impaction efficiency for the smallest particles was lower in the non-isothermal referencecase compared to the isothermal reference case. This is believed to havebeen caused by inaccuracies in the numerical method for the non-isothermalsimulation at Re = 380.Thermophoresis was not found to affect the particle impaction for thelargest Stokes numbers. For intermediate and small Stokes numbers the effect of thermophoresis depended on Λ. The particle impaction efficiency wassignificantly higher, both for the front side and the back side, in the ther-mophoretic simulations compared to the non-isothermal reference case forparticles with Λ = 1 and Λ = 100. The particle impaction efficiency forparticles with Λ = 1000 was lower, both for the front side and the back side,in the thermophoretic case compared to the non-isothermal reference case.