This study reports the development and evaluation of a web-based interactive writing environment designed for elementary school students. The environment includes three writing themes, "story pass on", "story chameleon" and "thousand ideas", to encourage reading comprehension, creativity and problem-solving skills of students. Three assessment mechanisms, "expert assessment," "self-assessment" and "peer assessment," are also designed to provide constructive comments to foster students to review and criticize other writers' essay, to enable students to review their own essay to find strengths and weaknesses in writing, and to encourage students to improve their writing skills. The writing environment comprises four functional modules--writing, assessment, tool and system management. The system was integrated with multilayer educational services platforms, which are designed to support the establishment of online social learning communities for K-12 students and teachers. The system logs and assessment results have been analyzed through the system usage over two years. The results reveal that students can improve their writing skills by participating in the writing environment, submitting many essays, interacting with other students online and reviewing other essays. The comparison result of early and late student writing also demonstrates the improvement of writing. Analysis of the assessment mechanism reveals that expert assessment and peer assessment do not significantly differ. It appears that the assessment criteria proposed in this study fit the needs of both the expert and elementary school students. It is convinced this writing environment effectively helps students to write with designed social interaction, creative writing themes, and reflective assessment criteria. (Contains 4 figures and 11 tables.)