The technique recommended to achieve adhesion of composites to the enamel and dentine in dentistry is incremental with gradual photo-activation. Bulk fill composites were developed to reduce the clinical time and shrinkage polymerization. There are two types of bulk fill composites according the viscosity. The present study evaluates the shrinkage and temperature change during the polymerization of a bulk fill composite of low viscosity inside a teeth cavity, prepared to replace teeth tissue lost by caries and fractures. Fiber Bragg grating sensors are used in this investigation due their great sensibility and small dimensions. The results reveal shrinkage polymerization of −863±168 με and temperature change of 12±7°C. The temperature variationis high and can compromise the pulp vitality.