在《大法鼓經》中，「不可說」是佛陀宣說「如來藏」思想中一個非常重要的側面。如來藏一詞就其文意並參考經典中的脈絡，應該理解為「處在受煩惱覆蓋而未發覺清淨如來的狀態」。「不可說」作為線索能夠指引修行者不以錯謬的知見認識如來藏，進而排除煩惱而親見如來法身。本文目標在於循此線索釐清如來藏思想的內涵。一切有情皆有如來藏，但這不意味著一切有情皆能信受如來藏之教學。對於根器不足者，要理解如來藏勢必有其障礙，因此需要適當的方便解決此一困境。本文爬梳《大法鼓經》中的義理，整理出佛陀為接引有情修習如來藏而施設的方法。以三輪體空為參考架構，初步可將經典內容區分成聽者、說者和法，三個面相。針對這三個面向一一說明如來藏的不可說，可以一層層地去除掉世人的錯謬知見。捨離對語言文字的執著，不繼續以世間的境界來思惟衡量如來藏，以上二點負面的排除初步展開了修行的大門。正面提升的部分有賴修行者對法生起信心，順著佛陀如實而說的教法修行朝向如來境界。隨著認知能力的層層轉進並搭配上修行，修行者最終能夠親見如來法身，以及徹底了悟如來藏法理。

In Mahabheriharakaparivarta sūtra, ineffability is quite an important approach to delineate ＂tathagata-garbha＂. Considering the context and the meaning, the term ＂tathagata-garbha＂ should be translated as ＂the life with tathagata covered by illusion＂. Furthermore realizing the ineffability may lead us to remove ＂shadow of illusion＂, and thus attend dharma-kaya. Through this way, I delineate the tathagata-garbha. Due to the different endowment, not all the sentient-beings can understand tathagata-garbha. To overcome the difficulty is the main purpose of this research. Basing on Mahabheriharakaparivarta sūtra, we can find the clues of ineffability which the Buddha left for us. For the novice of tathagata-garbha, one should abandon one's defective thinking, because it can not help to go deep into reality. Besides, we should avoid attaching to Buddha's literal meaning and faithfully follow the Buddha's teaching of tathagata. To approach to tathagata, we must practice the Buddha's teaching. By practicing, we can eliminate illusions and go deep into reality, the tathagata.