兩者均可表示“高”，但high通常表示具體的高，且主要修飾動詞；highly 主要表示很高的程度(意為“高度地”、“非常地”)，除修飾動詞(如 speak, praise, think of等)外，還通常用于修飾分詞或形容詞(尤其是那些由動詞派生而來的形容詞, 有時相當于 very much)。如：

He climbed high. 他爬得很高。

He speaks very highly of you. 他十分贊賞你。

His report is highly scientific. 他的報告是很科學的。

She is a highly educated woman. 她是受過很高教育的女人。

The Chinese ate a highly civilized people. 中國人是有高度文明的人。

At this news they were highly indignant. 聽了這消息他們非常憤怒。

The diesel engine is highly efficient. 這臺柴油機效率很高。

It was highly confidential. 這是高度機密的。

It's highly likely that he will succeed. 很可能他會成功。

They thought very highly of him. 他們對他評價很高。

A news commentator should be a highly literate man. 新聞評論員應當是受過良好教育的人。

The industry of France was not yet highly developed. 那時法國的工業尚未高度發展。

"The Rime of the Ancient Mariner" is a highly imaginative poem. “古舟子詠”是一首非常富有想像力的詩。

【注】在用連字符構成的復合形容詞中，通常要用 high。如：high-priced 高價的