In analyzing mutual funds’ ؛ performance, the funds’ ؛ return is typically deemed as a performance measure and this measure is not necessarily discriminated by the virtually investors’ ؛ realized return. This is despite the fact that depending on the investors’ ؛ timing strategy and the amount they decide to invest, the actually realized return may likely be different with the announced return by the fund, which is normally based on the buy and hold strategy. In the present study, we are going to answer how investors’ ؛ timing decisions in mutual funds may affect their returns and eventually their behavior. To do so, we have exploited the registered mutual funds’ ؛ cash flow turn over over 1390 to 1395 (2011-2016) and finally come up with conclusion that inefficient market timing by mutual funds investors’ ؛ have culminated in diminishing 2. 2 % their monthly return (30 % annually). This considerable negative effect can significantly alleviate the investors realized return even in outperforming mutual funds.