First, "I have read a paper from Japan Chemist saying that the oxidation rate of SO3 is 1/10 of Na2SO3." Why you do not cite this paper and believe that it's not reliable? 

Second, SO3 is the anhydride of H2SO4. The following reaction occurs:

SO3 (g) + H2O (l) → H2SO4 (aq)

I downvoted your question.

First, "I have read a paper from Japan Chemist saying that the oxidation rate of SO3 is 1/10 of Na2SO3." Why you do not cite this paper and believe that it's not reliable? 

Second, SO3 is the anhydride of H2SO4. The following reaction occurs:

SO3 (g) + H2O (l) → H2SO4 (aq)

I downvoted your question.

Van Huy Tran,    I agree with Yurii Geletii that this is a poorly written question.  It is unclear what you are asking.are you trying to compare the reaction rate of Na2SO3 with H2O and he rate of SO3 + H20 to form H2SO4?

SO3 doesn't oxidize. SO2 does, and the oxidation is usually done in concentrated H2SO4.

When Van speaks about SO3, I imagine that it is sulfite ion SO3-- and its oxydation in Sulfate Ion SO4--

One example : SO2 absorbed in water : SO2 + H2O --> SO3-- + 2 H+

When you neutralise with Soda you have a sodium sulfite solution Na2SO3

then you can oxidise (injection of Oxygen or air) and obtain a sulfate solution

With lime  or limestone milk instead of caustica soda, you transform calcium sulfate into gypsum by air injection in the bottom of the Scrubber  : CaSO3 + 0.5 O2 + 2H2O ---> CaSO4.2H2O

This is a principle of flue Gas desesulfurization sytem with production of synthetic gypsum . Try to find kinetics data linked to the process of SO2 gas treatment by wet scrubbing with lime, limestone or caustica soda

 In my answer please read calcium sulfite (CaSO3) instead of calcium sulfate

Thank you every one. My apology for my vague question.

I am working with a solution which contains both ammonium sulfite and ammonium sulfate (fertilizer). I am after the oxidation rate of SO3 to SO4 without any treatment (air injection, rapid mixing etc). The oxidation rate of SO3 to SO4 will affect the process that I am working on. The more SO4 in the solution is, the better.

I have searched and found out several paper discussed about it (oxidation rate). It is very low (almost zero). Maximum 25.E-09( mol/L.s.cm2) with the optimum concentration of SO3 in the solution.

 https://www.researchgate.net/publication/232361945_Kinetics_of_heterogeneous_oxidation_of_concentrated_ammonium_sulfite

Article Kinetics of heterogeneous oxidation of concentrated ammonium sulfite

You comments are as badly formulated as your original question. SO3 is water is sulfate. You probably mean SO3-, sulfite. The oxidation rate of SO3- to SO4-- without air injection, rapid mixing,etc will be controlled by mass transfer of O2 from air to you solution. 

 I agree with Yuri (see my answer)