A light emitting device includes a serially-connected LED array of a plurality of LED cells epitaxially formed on a substrate. The LED array includes a first LED cell, and a second LED cell adjacent to each other, and a serially-connected LED sub-array including at least three LED cells intervening the first and the second LED cells. Each LED cell includes a first semiconductor layer formed on the substrate; a second semiconductor layer formed on the first semiconductor layer; and an active layer formed between the first semiconductor layer and the second semiconductor layer; wherein the distance between the first semiconductor layer of the first LED cell and that of the second LED cell is larger than 30 μm, and one of the first semiconductor layers and/or one of the second semiconductor layers of the LED cells includes a round corner with a radius of curvature not less than 15 μm.