Nanotwinned metals have attracted widespread attentions recently, due to their unique overall properties, such as high strength, considerable ductility, enhanced work hardening and high electrical conductivity. The method of synthesized nanotwinned metals is an essential factor for influencing its application. To date, the direct-current electrodeposition technique is successfully used to fabricate bulk nanotwinned Cu samples. However, many parameters, such as the density of current, additive, the concentration of Cu²⁺, pH and temperature, influence the formation of nanoscale twins during electrodeposited process. To understand the effect of electrolyte additive on the formation of twins, in this work, gelatin with different concentrations was added into the electrolyte while other parameters are kept invariant. Bulk Cu with preferentially oriented nanoscale twins was synthesized in CuSO₄ electrolyte with different concentrations of gelatin. The nanotwinned Cu sample is composed of columnar grains with high density nanoscale coherent twin boundaries, most of them are parallel to the growth surface. It is found that the concentration of the electrolyte addition plays an important role in the twin lamellar spacing of the nanotwinned Cu samples, but has little effect on grain size. No twins or twins with micro-sized spacing are detected in electrodeposited Cu without the electrolyte addition. With the concentration of gelatin increasing from 0.5 mg/L to 5 mg/L, the average twin lamellar thickness of the bulk nanotwinned Cu samples decreased from 150 nm to 30 nm. Twin boundaries also grow longer in grains with the increase of gelatin. This is because that with the increase of the concentration of gelatin, the overpotential of cathode increases and nucleation of twins becomes easier, resulting in the reduction of twin spacing.