Abstract: A visual investigation of vapor bubble growth in subcooled upward forced convection boiling in vertical rectangular narrow channel （the gap is 2 mm） was performed. High speed camera was used to record the bubble growth with a speed of 5000 fps（frames per second）, and the picture size are 512×512 pixel. Experimental results about bubble growth in various work conditions （with variation heat flux and mass flux） were recorded; and based on the regression analysis, the exponential fitting relationship between the mean equivalent bubble diameter（D_e） and the bubble growth time was revealed. The results show that the exponential fitting can predict the mean bubble growth in narrow channel very well; but the coefficients of K and n, are quite different with that of conventional channel; and the K is less dependent on Ja number other than that of conventional channel, and the n has the value of wider range of 0.339～0.914.