两者均可表示“高”，但high通常表示具体的高，且主要修饰动词；highly 主要表示很高的程度(意为“高度地”、“非常地”)，除修饰动词(如 speak, praise, think of等)外，还通常用于修饰分词或形容词(尤其是那些由动词派生而来的形容词, 有时相当于 very much)。如：

He climbed high. 他爬得很高。

He speaks very highly of you. 他十分赞赏你。

His report is highly scientific. 他的报告是很科学的。

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

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

At this news they were highly indignant. 听了这消息他们非常愤怒。

The diesel engine is highly efficient. 这台柴油机效率很高。

It was highly confidential. 这是高度机密的。

It's highly likely that he will succeed. 很可能他会成功。

They thought very highly of him. 他们对他评价很高。

A news commentator should be a highly literate man. 新闻评论员应当是受过良好教育的人。

The industry of France was not yet highly developed. 那时法国的工业尚未高度发展。

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

【注】在用连字符构成的复合形容词中，通常要用 high。如：high-priced 高价的