## 1 Introduction  
Mairei Yew (_Taxus chinensis_ var. _mairei_) is a unique yaw variety of China. Recently, the studies on active compounds of Mairei Yew mainly have been focused on taxanes, especially paclitaxel, which has antitumor activity [1, 2, 3]. However, there are many active ingredients in branches and leaves of _Taxus_, such as flavonoids, polyphenols, volatile oils, polysaccharides, tannins, etc. [4]. Natural flavonoids from plants have antioxidant, anti-cancer, anti-inflammatory and heart protecting functions [5, 6, 7, 8]. Quercetin (QU), kaempfer (KA), ginkgetin (GI) and sciadopitysin (SC) are mainly the flavonoids in _Taxus chinensis_ var. _mairei_ leaves [9, 10, 11, 12]. The chemical structural formulas of QU, KA, GI and SC are shown in Figure 1. These four flavonoids have many pharmacological activities, including anti-inflammatory, anti-tumor, anti-influenza virus, anti-platelet aggregation, cardiovascular protection and blood glucose regulation, improving depression associated with epilepsy and immunosuppressive activity [13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. The traditional method for extraction of flavonoids is mainly heating reflux extraction (HRE). However, the yield of flavonoids obtained by HRE is low and extraction time is long [23]. Thus, it is necessary to propose a more effective method for extracting flavonoids. Lately, ultrasound-assisted extraction (UAE) has been applied for extraction of natural active ingredients, including flavonoids [24; 25; 26; 27; 28]. Because of ultrasonic vibration, high speed, strong cavitation effect and mixing effect constantly producing numerous internal pressure, reach thousands of atmospheric pressure microcavitation, and constantly \"blasting\" produce microscopic powerful shock wave on plant material. Thus, the matrix of plant materials was constantly eroded, and the active ingredients that were not belonging to plant structure were separated continuously [29; 30; 31]. In addition, cell walls' expansion can decrease the barrier of mass transfer between plant cell walls and solvents, thus accelerating the release of target components in a shorter time [32]. Temperature is an important parameter affecting the yield of target components in the process of ultrasonic extraction [33]. With the increase of extraction temperature, the solvent permeability and the solubility of target components increase, while the viscosity of solution decreases [24]. Thus, the increase of temperature can increase the yields of target components [34]. However, it is also reported that excessive temperature may lead to degradation of some thermal unstable components [33; 35]. In the known reports, the extraction temperatures used in ultrasound extraction are all less than the boiling point of solvents [36]. In fact, when the solvent reaches its boiling point, a part of the solvent going into the cell can be transformed from the liquid to the gaseous, which promotes destroying the cell wall, accelerating the separation of target components from the  
Figure 1: The chemical structures of quercetin (QU), kaempfer (KA), ginkgeetin (GI) and scidopitysin (SC). plant matrix. Therefore, the ultrasound extraction using boiling solvents may increase the yield of target components. [PERSON] et al. found that the yield of flavonoids from _Nephelium lappaceum_ L. fruit peel increased as the extraction temperature from 30 \({}^{\circ}\)C to 50 \({}^{\circ}\)C and the yields reached the maximum at 50 \({}^{\circ}\)C in the process of ultrasonic extraction [37]. [PERSON] et al. demonstrated that the yield of flavanone glycosides from orange increased as the increase of temperature with the range between 10 \({}^{\circ}\)C and 40 \({}^{\circ}\)C [38] and the maximum yield of flavanone glycosides was reached at 40 \({}^{\circ}\)C. [PERSON] et al. [39] found that the highest yields of flavonoids from Thai silk waste were obtained at the highest of 70 \({}^{\circ}\)C. In addition, [PERSON] et al.