We train a relative sentence readability estimator from a corpus without absolute sentence readability.Since sentence readability depends on the reader’s knowledge, objective and absolute readability assessments require costly annotation by experts.Therefore, few corpora have absolute sentence readability, while parallel corpora for text simplification with relative sentence readability between two sentences are available for many languages.With multilingual applications in mind, we propose a method to estimate relative sentence readability based on parallel corpora for text simplification.Experimental results on ranking a set of English sentences by readability show that our method outperforms existing unsupervised methods and is comparable to supervised methods based on absolute sentence readability.