Introduction: Children with specific language impairment (SLI) show a particular difficulty with grammar, which can be differently manifested in conversation and narration. Objectives: a). To explore the influence of narrative and conversational discourse in the grammatical performance of children with SLI, divided into two groups (4 and 6. years-old), and b). to determine whether the grammatical performance of 4 and 6. years-old children with SLI is different from children with typical language development (TLD) in conversational and narrative discourse. Method: The study included 21 children with SLI and 19 children with TLD. Three narratives obtained from a re-telling task and semi-spontaneous conversations were analysed according to two parameters: grammaticality and complexity. Results: Comparison between conversation and narration in children with SLI showed that the type of discourse affects grammatical performance. The way in which both types of discourse affect grammatical performance differs according to age. Grammatical performance of children with SLI tends to be lower than children with DTL in both types of discourse, especially in the narrative. Discussion and conclusions: Results suggest that narrative is more appropriate to address the ungrammaticality and complexity of sentences, due to its greater linguistic demand. Since conversation is a less demanding task, it can be used to support children with SLI that show severe grammatical difficulties.