Statement of Problem: One fourth of orthodontic patients can benefit from maxillary expansion but traditional expansion screws produce unfavorable heavy interrupted forces. A new spring- loaded expansion screw was designed which created light and continuous forces.Purpose: The purpose of this study was to compare the treatment effects and patients, discomfort with removable slow maxillary expansion and newly designed spring- loaded expansion screw.Materials and Method: 35 healthy Iranian children were divided randomly to two groups: group I (25 patients) treated by removable expansion appliance and group II (10 patients) treated by spring- loaded expansion appliance. The active phase of expansion was monitored and arch sizes of the upper dental arches (inter- canine, inter- premolar, inter- molar and arch perimeter) were measured with a caliper on casts monthly. The patients requested to mark the intensity estimation of their discomforts during wearing of appliance on questionnaires which comprised 12 statements. The scores of individual question were added up to obtain a total score. The independent t-test and Mann- Whitney U-test were applied to analyze the data.Results: There were no significant differences in both groups in the mean of arch size changes in each appointment ( p >0.05). There was no significant difference in both groups in terms of the mean of scores of questionnaires ( p =0.352).Conclusion: There was no significant difference in terms of patients, discomfort and arch size changes in spring- loaded and removable expansion appliances. Since the newly designed expansion appliance does not need to be activated by patients, it might be assumed a proper substitute for traditional expansion appliances.