Nri fund transfer to his mother account

If the transferred amount is loan/gift kind, no tax liability arise.

Thank to you for your valuable opinion

I assume that transaction is in nature of gift. There is no taxability on that amount to your mother. Income from investing that amount will be taxable in your mother's return. 

Since amount is big, execute a gift deed on plain paper signifying your intention to gift and her acceptance. No registration, stamping required. Execute in duplicate, and keep a copy in yours and mother's tax file. 

Preserve salary statements and tax paid statements for the money transferred - that can be asked in case of a scrutiny assessment.