A new process to fabricate gapless triangular micro-lens array (GTMA) optical film is realized in this study. Two-layer photoresists are used to define a triangular column array template. The upper and lower layer photoresists are AZ 4620 and AZ 9260, respectively. The two-layer photoresists form a higher aspect ratio than the individual photoresist does. This process includes a two-layer ultraviolet (UV) lithography, photoresist reflow process, Ni–Co electroplating and the hot embossing technique. After a triangular column array of two layers of photoresist is defined by UV lithography, the reflow technique is applied to melt the triangular column array into the shape of a triangular micro-lens array. With this reflowed triangular micro-lens array, Ni–Co alloy is electroplated and covered uniformly on the triangular micro-lens array to form the GTMA mould. After this electroplating process, a mould of GTMA is obtained, which serves as the primary mould. Next, with the passivation technique applied on this primary mould's surface, a secondary mould is obtained by applying the electroplating process again. This secondary mould serves as a master for the subsequent hot embossing process to replicate the GTMA pattern onto a polymethyl methacrylate (PMMA) sheet. The Ni–Co mould with a hardness over 650 Hardness of Vicker (Hv) is obtained. The stiffness and hardness of the mould play important roles in the GTMA hot embossing process. In addition, this PMMA-based GTMA film used as optical film offers a 100% fill factor and high optical coupling efficiency to improve the luminance. The optical measurement shows that this optical film with GTMA pattern increases 18.39% of luminance for a backlight module (BLM) of a liquid crystal display (LCD).