Need for reactors with big volumes is shrinking because of current market requirements that are inducing transition from massive to personal production. That transition could ease the introduction of microreactor systems instead of conventional reactors, at least for certain industries, such as chemical or pharmaceutical. Microreactor systems, that include nano, micro and millireactors, are characterized my better transfer of mass and energy when compared to conventional reactors, which in turn results in increased efficiency of processes. Additional improvement in mass and energy transfer is possible to achieve by adding static mixers within the microreactor systems, which affects the efficiency of the process itself. Aim of this paper is to develop and produce millireactors of the same geometry with different number of static elements, whilst also to test their effect on the conversion. Four millireactors have been manufactured using stereolitography, an additive production procedure. Tubular millireactor is one of the produced millireactors that serves as a benchmark, while three other millireactors have been different number of static mixers within the channels. Fenton reaction has been carried out within the millireactors, which rendered the colouring, Reactive Blue 182, colourless, while the sample at the exit of the millireactor has been spectrophotometrically analysed. Total conversion of the reaction has been calculated from the values of molar colouring flow-through at the beginning and the end of the reaction. Laminar flow was expected to manifest inside the millireactors. That was estimated using Reynolds numbers calculated for the tubular millireactor. Through performing the experiment, laminar flow has been confirmed. With lower flow-through volume, laminar flow is clearly seen, while in higher flow-through volume scenarios turbulent mixing of reactant entry currents is observed. By performing the Fenton reactions a relatively consistent increase of reaction conversions with static mixers in the millireactors is observed.