This paper investigates a new approach for Single Document Summarization based on a Machine Learning ranking algorithm. The use of machine learning techniques for this task allows one to adapt summaries to the user needs and to the corpus characteristics. These desirable properties have motivated an increasing amount of work in this field over the last few years. Most approaches attempt to generate summaries by extracting text-spans (sentences in our case) and adopt the classification framework which consists to train a classifier in order to discriminate between relevant and irrelevant spans of a document. A set of features is first used to produce a vector of scores for each sentence in a given document and a classifier is trained in order to make a global combination of these scores. We believe that the classification criterion for training a classifier is not adapted for SDS and propose an original framework based on ranking for this task. A ranking algorithm also combines the scores of different features but its criterion tends to reduce the relative misordering of sentences within a document. Features we use here are either based on the state-of-the-art or built upon word-clusters. These clusters are groups of words which often co-occur with each other, and can serve to expand a query or to enrich the representation of the sentences of the documents. We analyze the performance of our ranking algorithm on two data sets – the Computation and Language (cmp_lg) collection of TIPSTER SUMMAC and the WIPO collection. We perform comparisons with different baseline – non learning – systems, and a reference trainable summarizer system based on the classification framework. The experiments show that the learning algorithms perform better than the non-learning systems while the ranking algorithm outperforms the classifier. The difference of performance between the two learning algorithms depends on the nature of datasets. We give an explanation of this fact by the different separability hypothesis of the data made by the two learning algorithms.