在工作系統中液壓介質是用以傳遞能量的重要媒介，目前所知在系統中需達成的功能有：
It’s very important to transfer the energy of medium on the hydraulic system. In the meantime, we thank to get some working as follow：
(1)使潤滑元件，降低摩擦和減少磨損。
Lubricate the parts, down wears and frictions.
(2)傳遞訊號和能量。
Transfer the sign and energy.
(3)防止鏽蝕及散熱。
Resisted to become rusty and abstract heat.
(4)分離、傳輸並沉澱非可溶性污染物。
Separated and transfer and precipitate the non-soluble pollutant.
(5)填補原件間之摩擦間隙。
Filled in the space friction for parts to parts.
液壓介質主要性能的要求有以下幾點需注意
Attention to performance for the medium of hydraulic as follow
(1)黏度：黏度過大，將導致機械效率降低、溫昇加大、起動困難，甚至產生氣蝕並將使控制之靈敏度下降，致使參雜在油液中的空氣很難被分離出來。黏度太低，將致使容積效率降低及增加漏油的可能，並會導致控制精度下降及油膜變薄、磨損加劇。
Viscosity：Will to lead the temperature, efficiency and operated to no good when the viscosity too much or too low .
(2)抗磨性：抗磨性與黏度無關，主要是靠加入添加劑在表面形成潤滑膜而減少磨損的一種性能。
Antifriction：It’s major to fill some additive in the surface to became some velum for lubricate and less some wear.
(3)抗氧化性：抗氧化性是指油液之抵抗含氧物質，尤其是當與空氣接觸而起化學反應時仍可保持其性質，而不發生永久變化的能力。
Antioxidative：It’s a material to resist of oxygenous, especially together with air but still unaffected forever.
(4)抗熱性：抗熱性是指油液在高溫時抵抗化學反應和分解的能力。
Thermostability：To resist the chemical reaction and lysis.
(5)抗乳化性：阻止油液與水混合而乳化的能力稱為抗乳化性。
Anti-emulsibility：Stop the oil combine with water to cause emulsify.
(6)抗水解性：抵抗油液與水起化學作用的能力稱為抗水解性。
Anti-hydrolyze：Stop the oil combine with water to cause chemistry.
(7)消泡性：抑制油液泡沫之形成及可迅速釋放油液中之分散氣泡的能力稱為消泡性。
Make defoam：Stop the oil to form some foam and quickly disappear.
(8)防鏽蝕性：油液可阻止與其觸的金屬材料生鏽及受腐蝕的能力稱為防鏽蝕性。
Rust prevention.
(9)抗剪切性：是指油液之抵抗剪切作用，保持其黏度及其他與黏度有關性質於不變的能力。
Anti-shearing：To resist the shearing and maintain the original viscosity.
(10)與材料之相容性：液壓介質和所有與他接觸的材料之間不發生相互損壞和顯著影響性能的特性，稱為與材料之相容性。
Material Compatibility：The hydraulic of medium with others material don’t happen to lose and no influence the functions that we say it is compatibility.