OBJECTIVE To study the effect of humidity on emission of formaldehyde from blockboard. METHODS A 0.25 m3 of simulation chamber was used to explore the emission of formaldehyde from blockboard under different conditions of humidity. The temperatures of simulation chamber were kept at the degrees of 23 +/- 0.1, the velocity of ventilation of 1.0 L/min, the loading factor of (1 +/- 0.03) m2/m3. RESULTS The chamber concentrations of formaldehyde varied with times at the conditions of different humidities matched the hyperbolic logarithm relation. The chamber concentration of formaldehyde raised remarkably when the humidity were more than 60%. The chamber stabilization concentrations of formaldehyde varied with humidity matched the index exponent relation. Equation of curve was Y = 1.34 + 0.23e(3.30) x (R2 = 0.99). CONCLUSIONS The humidity could have an great effect on emission of formaldehyde from blockboard.