This study is conducted under the domain of Vygotskian Socio-cultural Theory (SCT) of mind and the notion of dynamic assessment to elevate the linguistic accuracy of EFL learners’ writing skill. 40 homogenous intermediate EFL learners from four intact classes were divided into two dynamic assessment (DA) and non-dynamic assessment (NDA) groups. As a pre-test, the participants were given writing topics during a course to assess their writing linguistic abilities. After analyzing and scoring the compositions’ errors based on four consecutive stages to error analysis and a model of linguistic errors (grammatical, syntactic, substance, and lexical), the type and frequency of errors’ sub-categories related to each category (n=516) was detected. Afterwards, the DA group went through intervention/mediation (interventionist dynamic assessment) based on their pre-test errors in five sessions, while non-dynamic assessment group received no mediation and feedback. The pre-test-mediation-post-test design (sandwich model of dynamic assessment) was used in this study. After mediation, as a post-test, both groups entered the second course of language learning and like pre-test their compositions during the course were analyzed and scored to see the effect of dynamic assessment on learners’ writing ability. Analysis of data through descriptive statistics and statistical analysis showed that the experimental group who received mediations, unlike control group, solved their writing difficulties extensively and outperformed the control group with a significant difference. In other words, application of DA as an alternative procedure to standard testing has a positive effect on both test performance and writing linguistic accuracy of EFL learners.