Abstract : Since 2014, some of the countries that were formerly belligerent of the Great War – most particularly France and UK – have organised a series of commemorations of the First World War, known as the ‘Centenaire’ (France) or the ‘Centenary’ (UK). We can assume that there is a strong link – that cannot let a historian indifferent – between those commemorations, collective memory and historical studies. Though studies about collective memory are numerous since the famous works of the French sociologist Maurice Halbwachs (Halbwachs, 1950), few of them are examining how collective memories are being expressed – maybe even transformed – on social networks on-line. In the case of the Centenary of the First World War, a set of questions can be asked: What is the on-line echo of the commemoration of the centenary of the 1st World War? What is the behaviour of Memorial/Heritage Institutions about the 1st World War on Twitter? How do they transmit information about the Centenary? Is there an influence of the English predominance on Twitter about the Centenary on how non-english-speaking twitter accounts are considering the 1st World War? Are there specific subjects that are discussed on-line? Which ‘temporalities’ are present in tweets when Twitter users speak about the Great War on-line?