Objective To investigate the characteristics and relationship of cognitive function and resting state functional magnetic resonance imaging(fMRI) amplitude of low-frequency fluctuations (ALFF)in childhood and adolescence-first-onset obsessive-compulsive disorder(OCD). Methods Twenty-four childhood and adolescence-first-onset OCD patients (patient group) and thirty-three age/gender/education-matched healthy controls (healthy control group) were enrolled in this study.The participants′ cognitive function was tested with trail making test (TMT), symbol coding, Hopkins verbal learning test-revised (HVLT-R), brief visuospatial memory test-revised (BVMT-R), verbal fluency test (VF), digital span test, maze test and Stroop test.Then a resting-state fMRI scan was conducted for each participant.Amplitude of low-frequency fluctuation approach was used to explore the diffe-rences of resting-state brain function between patients and controls, and analyze the significant brain regions by adopting t test.The cognitive functions of patients and healthy control group in the significant brain regions were compared by using Pearson correlation analysis. Results The patient group had lower scores than healthy control group in symbol coding, BVMT-R and digital span test, and the differences were statistically significant [(45.24±10.32)scores vs.(56.85±9.57) scores, (21.94±6.99)scores vs.(30.58±3.92) scores, (64.06±20.00) scores vs.(78.03±18.08) scores; t=3.96, 4.90, 2.50; all P<0.05]. Corrected by Alphasim, patient group showed significantly increased ALFF in the left and right temporal lobe, left and right posterior lobe of cerebellum, right middle temporal gyrus, but ALFF decreased in the area of left and right lenticula, left putamen compared with the healthy control group (all P<0.05 ). The patients group showed that TMT scores were positively correlated with the value of ALFF in the right temporal lobe (r=0.67), and color scores were positively correlated with the value of ALFF in the right middle temporal gyrus (r=0.53), but colour scores were negatively correlated with the value of ALFF in the right temporal lobe (r=-0.54); the healthy control group showed that BVMT-R and digital span test scores were positively correlated with the value of ALFF in the left temporal lobe (r=0.47, 0.41), but digital span test scores were negatively correlated with the value of ALFF in the right temporal lobe (r=-0.49), and colour scores were negatively correlated with the value of ALFF in the left temporal lobe (r=-0.35), while word and color-word scores were negatively correlated with the value of ALFF in the right middle temporal gyrus (r=-0.39, -0.36); the correlation coefficient above had statistical significance (all P<0.05). Conclusions The childhood and adolescence-first-onset OCD patients had cognitive impairment, including speed of processing, visual learning and memory, working memory; and the patients showed abnormal spontaneous neural activity at some brain regions.The cognitive dysfunction may be associated with abnormal spontaneous neuronal activity in childhood and adolescence-first-onset OCD. Key words: Obsessive-compulsive disorder; Child; Adolescent; Cognitive function; Functional magnetic resonance imaging; Amplitude of low-frequency fluctuation