It has been carried out the modification of Sandmeyer modification reaction by in-situ generated formaldehyde. The insitu generated formaldehyde has been performed via heterogeneous phase reaction between 6.40 mL (0.10 mol) CH2Cl2 as organic phase with 40 mL 6.0 N KOH-alcoholic solution as aqueous phase and 2.56 g tetrabutylammonium perchlorate as phase transfer catalyst (PTC) by 40o C for 4 hours. Furthermore, 5.9 mL (0.05 mol) guaiacol 1 is dropwised to add to the reaction mixture. After the addition of guaiacol 1 finished, then reaction is allowed to run for 1 hour, so that the white crystal of vanillil alcohol 10 can be produced in 25.3%. Followed by oxidation of vanillil alcohol 10 with mixture of K2Cr2O7/conc. H2SO4 solution as oxidator, and 18-crown ether-6 as phase transfer catalyst (PTC) in heterogeneous phase at the room temperature for 5 hours, can be afforded vanillin 4 in 6.2% (calculated from starting material 1).