In this study, mechanical, thermal, fire and morphological properties of thermoplastic composites filled with fire retardants (FRs) and microcrystalline cellulose (MCC) were investigated in detail. The composites were produced through the extrusion + injection molding methods using MCC, FRs and high density polyethylene (HDPE). Based on the composites dry-weights, 10%wt. of mono ammonium phosphate (MAP), ammonium zeolite (AZ) or natural zeolite (NZ) were incorporated into polymer matrix for improving reaction of fire of the composites, and their effects on technological properties were evaluated. MAP had a positive effect on tensile and flexural moduli of the composites whilst tensile strength, elongation at break and impact strength properties of the composites were decreased. The highest tensile and flexural strength values were obtained from HDPE/MCC/AZ samples, respectively, 22.13 MPa and 29.93 MPa. The minimum value for impact strength was obtained from HDPE/M/MAP5/AZ5 composite (1.68 kJ/m(2)) whereas the maximum value for impact strengths were achieved from control HDPE composite (5.15 kJ/m(2)) and HDPE/MAP10 (3.30 kJ/m(2)). Flexural strength and flexural modulus values of all HDPE composites were higher than the requirement by the ASTM D 6662 standard for polyolefin based plastic lumber decking boards. As the MCC content increased, the initial, maximum and final degradation temperatures of HDPE composites have not been influenced greatly. Thermogravimetric and differential scanning calorimetry analyses showed that the maximum rate weight loss temperature and the crystalline features of HDPE have not been significantly influenced by addition of MCC. Limiting Oxygen Index (LOI) level and charring increased when FRs were incorporated into polymer matrix. In addition, the burning rates of HDPE/MAP10 and HDPE/MCC/MAP5/AZ5 composite were slowed down by 64% and 62%, respectively. It was found that no burning in HDPE/MCC/MAP10 was observed composite during the horizontal burning test. Especially; HDPE/MCC/MAP samples can be used for building products because they meet the CC1 (plastic materials that have a burning extent of 25 mm or less where tested at a nominal thickness of 1.5 mm, or in the thickness intended for use, in accordance with ASTM D 635) class requirements of international building code which means a model building code improved by the International Code Council. In the light of obtained results, it was specified that use of FRs improved morphological, thermal and reaction of fire of the MCC filled HDPE composites.