Previous research indicates that the quality of texts is associated with greater syntactic complexity and the use of a complex and diverse vocabulary, that is, a more sophisticated and dense vocabulary (McNamara et al., 2010; Perfetti & Hart, 2001). Besides, there has been increasing evidence on the importance and effectiveness of teaching grammar when it is taught in a contextualized manner and connected with the important problems of writing (Myhill et al., 2012; 2013; Jones et al., 2012). This approach considers grammar as a meaning-making resource that supports writers to make appropriate linguistic choices in the creation of text. In Chile it has been found that students in 4th and 6th grade have difficulties in their written production, especially in cohesion, punctuation and vocabulary (Sotomayor et al., 2016; Gómez et al. 2016). It is possible that these problems are related to a lack of understanding of how the language works, particularly regarding the structure of the grammatical sentence (Hudson, 2001; 2004) and the characteristics and development of a quality vocabulary. The study proposes an intervention consisted in two Teaching Unit (grammar and vocabulary) composed by 10 lessons where grammar and vocabulary is teaching in a contextualized manner in the written production of narrative and expository texts. The aim is to evaluate the impact of this intervention in written production of a sample of 360 6th grade students and 12 teachers. These are divided into three groups (grammar, vocabulary and control) to determine the impact of each specific intervention. A pre and post writing test, intermediate evaluations in grammar and vocabulary and two verbal abilities tests (Woodcock, Muñoz-Sandoval, Ruef, & Alvarado, 2005) were considered. The study will also gather qualitative information: classroom observations, interviews with teachers and conversations with students about grammar, vocabulary and writing. In this communication we present the results of the pilot phase and the partials results of this study, in process at this moment.