Today's market needs require the industry to transition from mass to personalized production. Therefore, the need for large volumetric reactors is reduced. Millireactors are a potential replacement for standard reactors, at least in the fine industries such as pharmaceutical and chemical. Compared to the conventional reactors, millireactors have better mass and energy transfer and thus increased efficiency. By adding static mixers within the millireactors, higher productivity of the process could be achieved, as well as additional increase in mass and energy transfer. The aim of this paper was to develop and produce millireactor with different geometries of static mixers and examine their impact on conversion. Six millireactors were manufactured by the additive manufacturing process, stereolithography. Out of six manufactured millireactors one is a reference, tubular millireactor, and others have built-in static mixers inside the channels. Fenton's reaction was carried out within the millireactors and the sample collected on the millireactors outlet was collected and spectrophotometrically analysed. Conversions were calculated through molar flow at the beginning and at the end of the reaction. The values of Reynolds' numbers calculated for the tubular millireactor have shown that the flow is laminar for all used flow rates. During the reaction, the laminar flow was clearly visible at all lower flow rates, while for higher flow rates, strong mixing at the contact point of two reactant currents was noticeable. From the achieved values of conversions, while using the same velocities, it was noticeable that the millireactor that has the largest volume achieves highest conversion value, which is directly proportional to its volume. The greater the volume of the millireactor, the longer the retention time within the reactor, which results in higher utilization of the reaction. During the Fenton's reaction at fixed retention times, a normal trend of growth of conversion with a retention time could be observed. The expected distinct difference between the reference and millireactors with static mixers wasn't established.