This study aims to reveal how peer- and self-assessment practices influence the writing skills of 9th grade students. The study adopted mixedmethods explanatory design. The participants were 102 students attending a public school in Ankara. The quantitative data were collected through a quasiexperimental method, and qualitative data were collected through a case study. There were three groups of participants in this study: the 1st experimental group in which peer-assessment was carried out with 34 participants; the 2nd experimental group in which self-assessment was conducted with 34 students, and 34 students in the control group. The interventions lasted 7 weeks. Writing performance tasks and rubrics were used to gather quantitative data while a Semi-Structured Interview Form was used to collect the qualitative data. For the analysis, paired samples t-test, ANOVA, and content analysis were used. The findings revealed that there was a significant difference between pre-test and post-test scores of experimental groups in which peer and self-assessments were conducted whereas there was not a significant difference between the scores of the control group. The findings of ANOVA, the post-test results of the experimental and control groups showed that there was a significant difference between all groups in favor of the 1st experimental group in which peer assessment was applied. The qualitative findings of the study corroborate the quantitative findings. Hence, we can conclude that peer and self-assessment practices were effective both in the development of students' writing skills and on their attitudes and interests towards writing.