XTEA is a block cipher with a very simple structure but there has not been found attack even for half of full round version i.e 32-round version. In this paper we introduce a class of weak keys which makes a 34-round reduced version of XTEA vulnerable to the related-key rectangle attack. The number of such weak keys is about 2108.21. Our attack on a 34-round reduced version of XTEA under weak key assumption requires 262 chosen plaintexts and 231.94 34-round XTEA encryptions.