Low-temperature and self-reducible copper inks composed of copper–amino complexes and proper submicron copper seeds are successfully developed, which can increase the copper load of inks to achieve high performance thick and dense conductive patterns. During the heat treatment, the preset copper seeds provide heterogeneous nucleation sites for metallic copper generated from the decomposition of self-reducible copper–amino complexes. These fresh copper nuclei homogeneously attach to the copper seeds to activate their surface, which contributes to the connection and neck-growth between these submicron copper seeds to achieve high conductive copper patterns. The effects of the size of copper seeds and the ratio of the amount of copper–amino complexes to seeds on the electrical resistivity and morphology of sintered copper patterns are clarified, and the functions of the heat treatment temperature and holding time are investigated. The results show that the sintered copper pattern with a high metal load of 36.8%, prepared by heat treatment at a low temperature of 140 °C for only 15 min under nitrogen atmosphere, achieves a low resistivity of 11.3 μΩ cm. Furthermore, the sintered patterns maintain a high-qualitative surface morphology and favorable thickness, and also exhibit a strong adhesion to polymer substrates which will be advantageous for the fabrication of wearable devices.