Phononic crystals (PC) have been a remarkably active research field for more than two decades [1]. The principle of Bragg reflection on an artificial crystal-like structure leading to additional spectral (band gaps) and refractive (negative refraction, anisotropy) properties is scalable in any frequency range by a suitable choice of the crystal lattice constant. Recent works have demonstrated nanoscale PCs as a mean for reducing the thermal conductivity by a modulation of the thermal phonon propagation [2-4]. Such phononic engineered structures are promising candidates as efficient thermoelectric materials compatible with CMOS technology since the lattice thermal conductivity can be reduced without affecting the electrical conductivity and thus increase the thermoelectric figure of merit zT. In this work we present experimental results including: i) the fabrication of phononic crystals in 70 nm thick SOI and ii) integrated heater and sensor devices for thermal conductivity measurements. Finally, molecular dynamics simulations based on Green Kubo method will be presented [5]. [1] M.S. Kushwaha et al, Phys. Rev. Lett. 71, 2022 (1993) [2] J.-K. Yu et al, Nat. Nanotechnol., 5, 718 (2010) [3] P. E. Hopkins et al, Nano Lett., 11, 107 (2011) [4] J. Tang et al, Nano Lett, 10, 4279 (2010) [5] S. G. Volz et al, Phys. Rev. B, 61, 2651, (2000)