莎士比亚十四行诗第十八首（Shall I compare thee to a summer's day）是威廉·莎士比亚创作的一首著名诗歌，以下是其全文：
Shall I compare thee to a summer's day？
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimmed；
And every fair from fair sometime declines,
By chance, or nature's changing course, untrimmed；
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st,
Nor shall death brag thou wand'rest in his shade,
When in eternal lines to time thou grow'st:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.
这首诗以夏天的日子作为比喻，赞美了某人的美丽和永恒。诗人首先提问是否将对方比作夏天，然后肯定地回答，认为对方比夏天更可爱、更温和。接着，诗人描绘了夏天的短暂和易逝，以及世上所有美丽事物最终都会凋谢。然而，诗人接着说，对方拥有永恒的夏天，不会褪色，不会失去美丽，也不会在死亡中消失。最后，诗人以这首诗将永远流传，来保证对方的生命力。
这首诗是莎士比亚十四行诗中最著名的一首，以其优美的语言和深刻的情感广受喜爱。诗中的比喻和意象生动形象，表达了诗人对爱情的崇敬和赞美。