The aim of this study was to investigate the effect of boron based fire retardants (BbFRs) and fiber loading on mechanical, fire and thermal performances of impregnated spruce wood flour (WF) filled high density polyethylene composites. For this purpose, WF was impregnated with solution of boric acid, borax and their mixture and dried before composite manufacturing. The effect of leaching on the limiting oxygen index (LOI) levels of samples with BbFRs was also investigated. Based on the test results, samples with borax provided better mechanical properties compared to the samples with boric acid, and showed 19% improvement on tensile modulus for 40% fiber loadings compared to control samples. In addition, BbFRs increased LOI levels and charring, and reduced the burning rate speed by 50%. Presence of boron after leaching was shown with SEM-EDX and LOI tests. It appears that especially borax could have potential to be utilized in wood plastic composites as fire retardants. (C) 2014 Elsevier Ltd. All rights reserved.