In this study, the effects of basalt fiber (BF) amount and initial fiber length on tensile, flexural and impact properties of poly (butylene terephthalate) (PBT)-based composites are studied. The effect of BF amount on thermomechanical and thermal properties of the composites is also investigated using dynamic mechanical analysis, differential scanning calorimeter and thermal gravimetric analysis. The BF is used at concentrations of 5, 10, 20 and 30 wt% and with different initial lengths of 3, 6 and 12 mm under constant loading of BF (20 wt%). According to the test results, the addition of BF improves the tensile strength and thermal stability of PBT at 20 and 30 wt% loading. Flexural strength and elastic modulus increase steadily as the added amount of BF increases. The addition of BF favors crystallization up to 10 wt% loading. No remarkable effect of initial fiber length is observed on the mechanical properties of the composites.