It is believed by many educationists that the traditional assessment approaches are less effective in students' learning. The contemporary trend in education favours the practice of alternative forms of assessment for improved students' learning outcomes. However, in the context of Pakistan traditional forms of assessment are being practiced in the classroom, which is considered a hindrance towards the holistic development of the learner and for the overall quality education in the country. The main aim of this study was to explore a science teacher's purposes, practices, challenges and opportunities of implementation of alternative assessment, who has participated in AKU-IED's Certificate in Classroom Assessment Course to contribute towards the understanding of implementation of alternative assessments in developing country context and subsequently to improve students' learning. A qualitative case study approach was used to explore the research topic. A primary level science teacher of a community based high school in Karachi, Pakistan was the primary participant, while the vice principal, section head of primary, a science teacher and a group of students of the school, formed the secondary participants. The data was collected through classroom observation, interviews, document analysis, field notes and reflective journal. The study revealed that, the research participant used alternative assessment for variety of purposes such as to enhance students' learning, providing feedback and grading. Questioning was practiced for getting students' attention and promoting thinking skills. Self assessment was used for giving students responsibility of their own learning and performance assessment was practiced to provide students opportunities to demonstrate variety of skills. The opportunities for implementing alternative assessment included teacher's professional development, which developed her basic understanding and skills for planning and implementation of alternative assessment. The skills included developing open ended questions, designing objectives based assessment tasks, setting clear learning targets and criteria which are aligned with her instructions and the provision of clear criteria and rubric based instructions to the students. Moreover, the features such as supportive school culture and autonomy for teacher to develop own syllabus, were seen to facilitate the process of alternative assessment in the classroom. The alternative assessment practices were also seen to facilitate student learning through their involvement in the activities, motivations towards their work and responsibility of their own learning. However, the study also revealed limited use of alternative assessment, such as lack of provision of opportunity for students to improve their work based on timely feedback. The challenges were limited