The aim of this work was to synthesize Al2W3O12 and Y2W3O12 submicronic powders and study the effect of incorporation of these fillers on thermal expansion coefficient (CTE) and mechanical properties of high density polyethylene (HDPE) composites. Y2W3O12 was synthesized via co-precipitation by mixing reactants and modified reverse strike co-precipitation, to assess the influence of the method on agglomeration degree of powders. Both methods produced Y2W3O12 submicronic powders (0.60 0.70 um), with similar primary particle sizes and agglomeration degrees, as verified by different characterization techniques. Al2W3O12 was synthesized via co-precipitation by mixing reactants and normal strike method, using two distinct tungsten sources and the assynthesized precursors were microwave dryied. The effects of synthesis pH (4.2 and 6.0) on final agglomeration state of the as-synthesized powders were assessed. Al2W3O12 submicronic powders (0.22 0.87 um) were synthesized by these routes. Co-precipitation by mixing reactants at pH=4.2 led to Al2W3O12 powders with smaller agglomerate sizes (0.22 um). HDPE composites were manufactured by micro-compounding, using Al2W3O12 and Y2W3O12 powders with similar particle sizes (0.70 um), at volume fractions in the range of 0.001 0.011. HDPE/Y2W3O12 composites presented CTE reductions of 20 25 porcento compared to neat HDPE, in the temperature range of 25 70 Celsius degrees, while HDPE/Al2W3O12 composites displayed reductions on CTE of 7 8 porcent. Young s moduli of composites were increased 11 32 porcent nd 21 porcent when filled with Y2W3O12 and Al2W3O12, respectively, whilst thermal stability of HDPE was preserved for all composites.