A new method to obtain composites of phenolic resin reinforced with microfibrillated cellulose with a wide fiber content was established and the mechanical properties were evaluated by tensile test. A linear increase in Young's modulus was observed at fiber contents up to 40 wt%, with a stabilizing tendency for higher fiber percentages. These results were ratified by measurements of the coefficient of thermal expansion (CTE) relative to fiber content, which indicated a strong thermal expansion restriction rate below 60 wt% fiber content, indicating the effective reinforcement attained by the cellulose microfibrils. The low CTE achieved of 10 ppm/K is one of the important properties of cellulose composites. © Springer Science+Business Media B.V. 2008.