I joined as a junior in QA dept then grew to manager,day after day the products of the company get larger and I am unable to conduct all experiments and tasks lonely. So what I need to ask my boss to dedicate someone could assist me and achieve the required work at high level .could anyone help me in writing this email properly and convincingly??
Before doing so, track your tasks and progress to ensure you have an accurate record of what you are doing and not doing. Otherwise, "I cannot keep up" becomes opinion and is easily refuted by management.
Second, consider automation or other improvements. If you can show efforts to do your job better, arguments for new staff are much stronger.
If both these avenues are exhausted, talk to your boss. Start with the facts (both the above) then suggest help. If you start with the request for new staff, it can sound like frustration rather than a rational argument and bosses are good at dismissing non-rational discussions.
You do not elaborate on your organization but your boss may not have the authority to hire or assign new staff. He or she may need to request to his management and, even if the case is sound, budget constraints or dozens of other reasons may mean the request is denied.
In the meantime, if you feel like you cannot achieve everything in the allotted time, tell your boss everything you have to do and ask him/her to select what to do. Bosses are good at prioritizing issues like this. It also ensures any last focus or priority changes are considered.