I use cash method on my tax return, can i deduct all inventory cost at the same tax year of purchase instead of waiting the inventory is sold. If so, should i use the cost of goods sold form, or deduct them as "Other deductions"

If you don't keep an inventory, you can use the Other Deductions category.   Do I have to keep inventory?

Under the cash method, I can deduct inventory as other deductions or "COGS",

Under accual method, i can only deduct inventory as on "COGS",

Am i correct?

A couple of comments regarding your questions:

• Accrual method of accounting gives you a better picture of your business; in general, no highs or lows
• With the accrual method of accounting, you don't "deduct" inventory.  You track beginning inventory, purchases and ending inventory which then results in CGS.
• If you are eligible to use the cash method and not track inventory, you would deduct the costs (purchases) as noted by @DawnC either as "other expenses" on line 27a of the Schedule C (assuming you are filing this, however, there is no indication of what software you are using), or you can reflect it on Schedule C Part III on line 36.  Using this method you need to also show zero beginning and zero ending inventory.
• This is your 3rd post on this question.  

Thank you so much! If i cash method, can i still use accual method for inventory only meaning that i will use actual beinning inventory + purchase - actual ending inventory (i don't want to deduct full inventory cost, but i want to cash method for my tax return)