## 1 Introduction  
Since ancient times, plants have been used to treat various ailments due to their antioxidant activities. According to [PERSON] et al. [1], numerous medicinal plants have been investigated for their antioxidant properties. The active compounds present in natural antioxidants; such as phenolic compounds, flavonoids, folic acid, carotenoids, benzoic acid, andtocopherol are secondary metabolites of the plants that play an important role in the body's defense system [2]. It has been reported that the active compounds found in phenolic or polyphenol compounds (caffeic, benzoic acid, and chlorogenic acid) and flavonoids (catechin, rutin, and quercetin) are potent antioxidants, and they have been identified as metal chelators and free-radical terminators used to prevent harmful reactive oxygen species (ROS) from damaging tissues and cells, such as DNA, proteins, and lipids [3]. To date, Melastomataceae is one of the natural plant sources that has gained attention within the scientific world due to its significant therapeutic value. It is one of the most abundant and diversified groups of pantropical families, having 4200 to 4500 species in 166 genera and approximately 1000 species predominantly cultivated in tropical Asia [4]. In Malaysia, particularly, the genus _Melastoma_ consists of 22 species, 2 subspecies, and 3 varieties, which can be distinguished by the color of their flower petals, such as _Melastoma malabathricum_ (dark-purple magenta), _Melastoma decmfidum_ (white), and _Melastoma hirita_[4; 5; 6; 7]. Studies have reported that members of this family have been used by practitioners of traditional medicines to treat diarrhea, pox scars, inflamed wounds, gastric ulcers, and diabetes [8; 9; 10]. The phytochemistry of these plants has revealed numerous secondary metabolites or bioactive compounds, such as flavonoids and phenolic compounds [5]. Moreover, previous reports have shown the isolation of several flavonoids from _M. decmfidum_ yields naringenin, kaempferol, kaempferol-3-O-D-glucose, and kaempferol-3-O-[2'6'-di-O-p-trans-coumaroyl], which are responsible for their antioxidant activities [11]. In addition, due to the fact that plant extracts usually occur as a combination of diverse type of bioactive compounds, the identification and isolation of active constituents in plant extracts are commonly achieved using the application of spectroscopic or chromatographic techniques such as high-performance liquid chromatography (HPLC) and gas chromatography mass spectrometry (GCMS). Optimization of the antioxidant compound is a crucial step for a variety of applications, especially in industrial fields such as pharmaceuticals. To date, the mixture design experiment (MDE) has been one of the most powerful tools used as a systemic statistical approach and an effective method of determining the proportions of variables (ingredients) of a blend. In addition, it is used to minimize the number of experiments and, at the same time, identify the effect of the simultaneous interactions between the variables and the target response [12]. The MDE has provided satisfactory results for the optimization of the combination procedures, and it has been successfully applied in scientific research and development and in real-world problems. For instance, [PERSON] et al. [13] successfully optimized the amount of fermented soy flour extract solvents to obtain maximum phenolic and antioxidant activities using an MDE. Despite numerous studies of MDEs, the simplex-centroid mixture design (SCMD) experiment was used in this study to determine the optimum proportions or combinations for a target response of three different Melastomataceae leaf species (_M. malabathricum, M. decmfidum_, and _M. hirita_) extracts for the optimum antioxidant activities of phenolic compounds and optimum ABTS (2, 2'-azino-bis (ethylbenzthiaazoline-6-sulfonic acid)) radical-scavenging activities.